I always used to have nock high, no matter what. The game changer for me was when someone who knew what they were doing asked me if I had a callous on my 4th finger, string hand. I said yes, I always do. He said I have a high elbow when I draw, causing my string hand to torque on the arrow downward, causing it to rebound off the shelf. After switching to a rest (which I now use on every recurve & longbow), and working on holding mostly with my 2nd and 3rd fingers, that has been corrected.
As for brace height, I was told that a high brace makes a shorter power stroke, thus imparting less energy into the arrow and making it stiffer. And shallower brace height keeps the string attached to the arrow a little longer, causing it to hang on a little longer and not flex freely until it comes off the string. I am no authority on these things though.