IS ZERO REV TOO LITTLE FOR AN SPAN OF 5 AND 5 1/16

+1

Keep trying, keep experimenting, until you find what's right for you. I just completed the same test, and have a much more comfortable and consistent release, between various balls, and with more ability to vary my release. It's a great journey and you learn a lot in the process. Don't let anyone tell you what "your" numbers are, and/or that a small change doesn't matter. For some of us, (I believe based on lack of flexibility), a small change makes all the difference.

Tried 5/16 feel good for awhile then feeis like squeezing it then tried 1/8 felt like more control

Don't forget that if your thumb shrinks slightly, it takes more pressure to hold on. You should add tape or do something similar in order to make your thumb fit that hole. Unless you take into account temperature, humidity and your diet (salt or beer can make your extremities swell), any reverse might feel like it's too much, when your thumb and the hole do not match.

When I had a 5"/5 1/8" span, I used 3/8" thumb reverse. BUT I needed to add/subtract tape much more often than now when I use 0" reverse and use a 4 3/4" span.