Do what works best for you. I can’t do the PRS type position, and so my thumb while in a relaxed position for me kinda drapes over a little bit. And, I learned not to fully grip with my whole thumb but more lock my hand muscles but not flex my thumb at the tip.Awesome pics. I was playing around this weekend with thumb position and trying to see what reduced the tension the most and gave me the best control. I was almost correct in hand placement!!!!! Now I can get it right.
Keeping as many muscles relaxed as possible is what I learned to do. I found I was clenching my core, butt and hamstrings even in prone. Learning to relax cut reticle wobble and absorbed more recoil.