The single most important part of stock fit is the shooter. If you cannot mount a gun consistently in the same place, time after time, no one on earth can make a stock to fit you. I cannot count the number of times I have been asked to look at a stock fit only to find that the shooter does not mount the gun in the same place twice. One moment the eye is to the left of the rib, next to the right and with huge variation in the amount of eye visible over the rib. In such a case the only option is to tell the shooter to go away and learn how to mount a gun. Note that I am not saying mount a gun correctly but consistently. If the style is poor but consistent it is still possible to make a stock to fit. Better of course that the style is also correct but there are very many excellent shots who sport terrible style. Inconsistency is the killer.
The key is practice. Pick up and mount your gun at every opportunity, until doing so is as natural as breathing. The gun should come smoothly into place without conscious thought and should drop into the same place every time. All focus should be on the target and the gun should align itself with your eye naturally. All humans have the born ability to point. If you look at the target you are able to point the gun without any attempt to align it with the eye. Ideally, your stance would be open and relaxed with head up and eyes fixed on the target. The gun should come to the face and pull back smoothly into the shoulder at almost the same instant. Lifting the gun to the shoulder and dropping the head down onto the stock does not facilitate natural pointing. Try it. Point your finger at something whilst simultaneously dropping your head. You will immediately see that it is impossible.
There is no point investing in a custom stock unless you can mount consistently, shot after shot after shot. Take the time to practice your gun mount and you will see a huge improvement in your shooting ability. Only when this is achieved should you consider a custom stock.