There is no exact measurement quite the reverse.
In particular just cutting something off the back throws the cheek into a different place, usually changing the relationship so the eyeline and the sightline are also changed. Pitch is another factor that changes with stock length and sightline. Jeff Cooper is a great fan of a very short stock, Tom Hayes used to say the longer a man has been shooting the longer he likes his stocks. What's the sighting system including eye relief if scoped? Varies with recoil as well.
Maybe somebody around you is good at stock fitting, usually shotgun stocks but some people are good with shotguns AND rifles - some aren't.
Doesn't mean don't do it, people do it all the time - you might cut and try taking a little off at a time - you can do a final adjustment with thicker or thinner recoil pads and slip on recoil pads. Notice that even the perfect fit might vary with the seasons and different clothes.
