Thanks to fourunder, the even that is the grandparent here was so successful that two years later, we decided to repeat it -- but this time with 73 people (!) and with the decision that the beef was so terrific (again, thanks to fourunder) that we should have _more_ beef per person (!!). So this is how I found myself yesterday with two monster standing rib roasts: 7-rib roasts at 18 pounds a piece. As others have observed, the challenge with this much beef is that it's very hard to forecast how long it will take under low-and-slow -- and given the shape of the cuts (19 inches long, ~10 inches wide and ~8 inches tall), I had a hunch that it would be quicker than 20 minutes per 18 pounds (thanks again, fourunder, for noting the importance of shape in cooking time when the roasts get large). And from my last experience (see above), and from fourunder's findings over the last few years, and from TomMN's experience (thanks, Tom!), I knew that a longer rest wasn't something to be too concerned about -- so I decided to err on the side of caution and get the beef in at 11a for an 8:45p scheduled plating.
The first hiccup was the oven: as it turns out, it wasn't quite as deep as I had anticipated and I was unable to put the roasts side-by-side. Fortunately, it was sufficiently wide that I was able to put the roasts on separate racks, one above the other with the roasts cross-wise. (That is, if you were counting the ribs, you'd count from left to right of the oven, not from front to back.) I didn't have to resort to the creativity of Katie808 (though her experience was definitely helpful, as she had an identical amount of beef).
I calibrated the oven, and it seemed to be running a tad hot, so I set it to 200* (instead of 225* that I had two years prior). I also felt that I could set to 200* and, if things were too slow after a few hours, bump it to 225*. The one problem was that, based on the shape of my temperature probe and the clearance between the bottom rack and the top rack, I couldn't quite fit the curve of the probe in the necessary space. I did the best I could by stringing the probe up through the rack above it (a mistake -- read on). With the bottom roast in place with an awkward temp probe, I put the second roast in on the rack above it (where the probe had just enough clearance) and got things going.
Within a few minutes, it was clear that something was wrong with the bottom roast: it was getting way too hot, way too quickly. Both roasts went in at roughly the same temp (57* for the bottom roast, 54* for the top roast), but after 30 minutes, while the top roast was exactly where I would expect (59*), the bottom roast was showing 88*. After a little bit of freaking out that the oven might be highly asymmetric, I looked more closely at the bottom probe, and it was clear that parts of the probe sheaving were coming into contact with the rack itself. I opened the oven, pulled the probe and re-placed it to avoid the contact, but the temp was still too high. At this point, I got a second instant read in the bottom roast, but I couldn't get it in nearly as deep as it needed to be -- the reading was too high, but I wasn't convinced it was meaningful. I was obviously trying not to fiddle around too much -- I didn't want to keep opening and closing the oven door -- but on the other hand, if the readings on the bottom roast were correct, I potentially had a serious issue on my hands. Finally, just before 1p -- two anxious hours after the roasts went in -- I pulled the probe entirely, and put it in at an entirely different angle such that it was deep in the roast, but could make it under the clearance of the rack above it. The reading was there 79* (when the top roast was 88*) and I breathed a huge sigh of relief. At that point, things went smoothly. I could see that the top roast was a bit ahead of the bottom roast (which is what I would expect, with heat rising and all). Just because I know that I'm not the only beef nerd around here, a graph of the temps over is attached -- including the bad early readings.
My anxieties then shifted to the fact that the roasts (true to my fears) would be at 120* (my target temp) at ~3p and have a long, long rest before their 8:45p plating. Heightening my anxiety was that resting the roasts in the oven was a non-option: others were going to need the oven for sides and starters (it's a four course meal), and the oven didn't go down as low as 140* regardless. So, and again thanks to fourunder and the advice that I used so effectively two years ago, I bundled the crap out of the roasts: three coats of tin foil, and several heavy blankets tightly wrapped. Thanks to some of the kids of the host house of the party (who, it must be said, absolutely loved wrapping the roast), I found the warmest spot of the house: the floor of one of their closets.
And now we get to the only thing that I wish I had done differently: because one of the roasts was not exactly level, juice accumulated and ultimately got through the aluminum foil in the absurdly long rest. This meant the blankets ended up with some roast beef juice on them -- and I would recommend that anyone who is going to be resting for more than two hours go to absolute absurdity on the tin foil and consider five or more layers. Certainly, it doesn't hurt...
At 8:15p -- five hours after the roasts had started resting -- the oven was available for my final heatup (250*) and blast (500*) the roasts were both reading 118*. I was a little panicky, but assured myself that I had had the 5-10* of carry-over, and that temps had probably started to fall in hours 3 and 4 of the rest. Once the blast was over, we took out one of the roasts, cut it down the middle, and it was immediately clear that -- once again -- fourunder had nailed it: it looked and tasted absolutely beautiful. Once again, many were full of praise for the beef -- with again several saying that it ranked among the best beef they had ever had.
So, moral of the story is that you can indeed rest the beef for a long, long time -- and if you are in for a long rest, count on using a ton of aluminium foil and find as many blankets as you can. I also think I wouldn't allow _quite_ as much time as I did (I was planning for a worst case of a 7 hour roast time) -- if your roast is simply many ribs but otherwise normally wide and high, I don't think cooking time is affected _that_ much; it probably adds a constant (e.g., one hour) rather than a multiplier.
And, of course, thank you fourunder and all of the other participants on this thread! fourunder, you are the patron saint for slow-and-low standing rib roast!
Just had a huge success following your instructions, fourunder -- thank you! Two standing rib roasts: one 7.5, one 6.5 (three ribs each) for ~40 people as part of a sit-down meal. Brought them up to room temp, rubbed them down with ample salt and ground fresh pepper, and then in at 225* with a temp probe. Based on the numbers I had seen here, I was fully expecting 3.5 to 4.5 hours (or more) to hit fourunder's 118*, but to my shock, it as at 118* after a mere 3 hours! By the time I pulled it, that site was at 121*, with readings ranging up to 129*. I wrapped both roasts in tin foil (triply wrapped) and -- per fourunder's loving instructions -- moved them away from all drafts and covered them with a blanket. This was throwing off my plan a bit as it was 5p when this happened, but we did not intend to be eating beef until 8:30p. After some frantic reading here, I inferred (correctly, I guess?) that a longer rest period would do no harm -- so the two roasts rested for nearly 2.5 hours (on top of it all, dinner was running late). At the point that they went back in, an instant read was giving me 129*, which matches roughly what I expected the carry-over to be given that the temp was ~124* when I initially pulled it. Then it was 250* for 30 minutes and the 500* blast for another 8 or so. Interestingly, during the entire warm-up and blast, the probe never budged about 129* -- pretty clear that the center is not being warmed by that process! Anyway, pulled the roasts out with some apprehension that they would be either too rare or otherwise too cool -- but the beef was terrific. Biggest problem (such as it was) was that the beef had to wait on the potatoes and therefore was a tad cooler on the plate than I might have liked (we couldn't warm the plates, which would have made a difference there). Many, many people were saying that it was the best beef that they had ever had -- and I think that that might be true of me as well. I was terrified of screwing up dinner for 40 people, but thanks to fourunder's clear instructions and patient clarifications, it was a huge hit! Thank you, everyone -- and fourunder especially!