a rule of thumb is that the butt to grip should be the same as distance from inside elbow to grip... ie instead of sticking butt in shoulder, stick it in elbow cleft and see if hand is stretching to reach grip or has too much slack.
then i set cheekpeice to the height that is comfortable, in the main position, then try and set the scope to that in terms of height and eye relief, but if you're doing HFT then most prefer the scope as close to the barrel as poss for trajectory reasons... but still check the eye relief and head position. Then i check the butt is allowing a neutral muzzle, ie it's not pushing it up or down when all feels natural... if it's pushing muzzle down, raise, and the opposite follows if it's pointing high.
if in any doubt, i push an adjustment to each extreme and see what each feels like, and go from there.
you should get close after a session, but it's not unknown to be tweaking for a while... the scope is the biggest pain to move, so spend some time there.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep