Thank you both for your advice. I've been researching and I think I got it. Please correct me if I'm wrong.
I'll take the roast out of the fridge and let it sit for about 1 hour to get to room temp. Then I'll put a rub on for about 1 hour. ( olive oil, salt, pepper, rosemary and garlic slices inserted into the roast. Cook at 400 for 30 minutes then turn the oven down to 325. Approximately 20 minutes per pound. Use a thermometer near doneness. About 140 it should read. Remove from oven and foil tent for 30 minutes for carryover.
Bake in a shallow pan. Don't cover. Do I add some water to the pan? I want to make a beef dip. Any suggestions? Because I'm serving a large crowd how do I keep the sliced meat warm without cooking it more. I keep telling myself I'm only cooking a big ole hunk of meat not splitting an atom. Lol
Thanks for the advice

i have a 12 pound sirloin tip roast that originally was going on the BBQ. But it's going to rain on Saturday. My questions are:
Do I cook on convection bake or bake?
What temp? I read anywhere between 200 to 325 degrees. 375 for the first hour.
What do I do for flavor? Rub or marinade ?
How long?I've never used a meat theometer. My late husband was the master at cooking. So I'm kinda clueless. Do I stick the thermometer in at the get go or only to test for doneness?
I'm having a b-day party for 20!!! Planning on a beef dip. Any great recipes will be appreciated. I thought it would be easy to cook a roast but know I'm getting scared. Please give me some answers to the conflicting material I've read
Thank you