My experience is that once you've got the rib picture you feel comfortable with the rest is just fiddling. Xmm here or there makes no difference. I can see the benefit of an adjustable stock if it's cheaper than replacing a stock that is way off but no more than that. I've had guns that feel great when mounting them but unable to hit a bears arse with them even after spending loads of time adjusting/changing stocks etc and other guns that seem to be great even if I've fitted a sporting stock for trap. Like everyone else I've spent hours experimenting only to find I end up back where I started with the pound coin on the rib, As taught when I started shooting. I was also given that clay shooting has to be treated more of an art than a science and it's nearly impossible to repeat the same thing all the time and Unlike rifle shooting, the sight picture changes all the time.
Try this:- mount you gun and point down towards the floor at say 45deg, how much rib can you see? Now do the same up. I bet it's different.
In my mind it's the target picture as you pull the trigger that is important, the only way to do this is by learning them subconsciously/muscle memory/ feel/ magic/ (take your pick as to what the current sports psychologist wants to call it) that from what I can deduce from talking to the best shooters is usually referred to as; see it shoot it.The trouble is that we all want to beat the process, start to think consiously about it and want turn our guns into some sort of Excalibur preventing most of us from progressing.