First, I suspect tabbouleh is like meatloaf--everybody does it somewhat differently. I learned from a woman of Lebanese parentage and even thought I know most Lebanese have a heavy hand with the parsley, she used about the same ratio as your recipe. She soaked the bulgur for two hours and squeezed it dry, handful by handful. Also, she always used cinnamon and garlic salt (rather than raw garlic). She used sumac in other recipes but never tabbouleh.