You have to figure too, your personal geometry into the equation. The size and shape of your face, your vision and how you have to get down on the stock to get a decent sight picture (aging eyes can really change shooting form), length of neck, width and thickness of shoulders, natural length of pull; all can have an impact.
Most off the shelf rifles and most kit guns are built to some sort of medium "fits all standard" which may or may not fit any specific shooter. A custom built gun should take all of those factors into account
Most of my long gun builds over the years have been french-style smooth bores. Most have a lot of drop and a pretty well rounded heel. Most were also pretty light for bore size almost like a fowling piece. I found that matching measurements taken from originals created problems for most guys. The 18th C. French-Canadian woodsmen and voyageurs must have been shorter armed and thicker chested, which is logical. and the short butts worked for them. I found adding an inch or two, to the stock length made a world of difference to my modern, taller, longer armed, and generally thinner friends.
you might try temporarily adding some padding to lengthen the butt and see if it changes things. Even if it is not recoil absorbent I suspect it may help.