When I went in to get my stuff redrilled, he looked at my measurements for my span. When he saw that it was 4 13/16 by 5, he said 3/8 reverse. My thumb is really flexible though. Which goes along with that chart (with what I could read of it).

It all depends on your driller and his beliefs on pitches. Coach Jim is a proponent of forward pitch. Years ago, it was tons of reverse and bevel the heck out of it. Hard to get some of the older drillers into thinking differently.

When you say there should be no pain in the fingers when you have correct pitch and such, does that mean there's no weight on the fingers?

Gah...sorry...
Can't find the edit button.
But could someone possible post a picture of a proper hand position in their bowling ball?

No, there's a difference. You will have weight in your fingers, but there should not be any pain when holding the ball in your hand or releasing it.

What about feeling fatigue/soreness in the thumb after 6+ games? I'm still fiddling with my fit and can't seem to find anything. I got the span figured out but I'm stuck on the pitch. I'm at 1/4 and 3/8 forward(different balls). Squeezing both. But at 3/8 with less bevel sometimes it hangs, sometimes it doesn't. One time it fell off my hand during my down swing. I'm very uncomfortable with the less bevel so I think I'll stay with my normal amount and go forward if need be. I've had some rubbing on the right side of my thumb at 1/8 right lateral so my driller is taking me to 0. Not sure how this will affect the way I grip but maybe it'll maybe it easier to find something as well.

Changing to 0 lateral might help. Some people are fidgety when it comes to thumbs. I used to be like that but then I relaxed a bit on it. As long as I don't have to grab it, I can learn to live with it.

The soreness and fatigue probably can be attributed to feeling tense about your grip. I know it's not cheap, but changing one thing at a time will give you a good feel eventually.

That is telling you to either put tape in the thumb, or if you have so much tape that you can't put any more and still get your thumb in the hole that you need to go 1/4" more forward.

The only reason that the ball doesn't fall off your thumb after that is you either went and put enough tape to prevent the thumb from falling out, or you are then squeezing it. If you are squeezing the thumb then that is why no bevel is hurting your thumb, and is one of the reasons to have less bevel so you will feel uncomfortable when you are squeezing and force you to fix your thumb so you don't have to squeeze anymore.

When the ball fell off my hand I decided to just start squeezing again. That ball had less bevel and it seemed like I was in a position where adding tape would cause hang because that was occasionally happening before my thumb shrunk a good amount. One of my other balls that has 3/8 forward has a little more bevel(from needing more room in the top of the hole) but it seems to stay on my hand better but with a bit more grip pressure than normal, although I can do it without bending the thumb. This is also with tape on the back of my thumb which I wasn't wearing at the time the other ball fell. So either it's the less amount of bevel that's better or the tape, or both. Maybe with that amount of bevel I could be at 1/2 forward. My worry has been once I get to the point where the ball will actually stay on my hand with minimal grip pressure, how do I expect it have a clean release as well. I think tape on the back of my thumb is key to that but I still wonder. I'm also starting to second guess my span which could be too short which is why I keep having inconsistencies in pitches and having to go so far forward at that.

Not to sound trite, but this the bulk of your problem; worrying, which means you're bringing your head into the equation. The only way I know of that you can break that habit is to practice so you can become comfortable with your grip instead of thinking about it.

As I've said before, when your brain starts throwing the ball, that's when you get into trouble.

Ever since I started making changes to my grip, the thought of how it's going to stay on my hand and how it's going to come off has always been in the back of my mind especially on the approach. I think I know how to control my grip(I've practiced it plenty) but it's just weird to me that a ball would stay flush on my hand AND release at the correct time. I feel I should be able to grip the ball a bit without bending my thumb and be fine. It's quite frustrating that I haven't figured it out by now but I should be close.