What is your drop at the comb from the sight line?
To measure it on your own body you can use a pair of calipers and go between where your cheek bone is, and the middle of the corner of your eye. Or, find a place on the stock that lets you see the sights clearly, and mark the spot where your cheek bone rests comfortably. It will be a zone between the front and back, Now take a straight edge between the front and rear sights, and extend that back through the butt. Measure that drop at the comb. that's the drop you need.
Now, shoulder the rifle and wee where your cheek feels most comfortably placed on the stock in the position you expect to shoot the gun the most (for most that's standing position, or offhand). Pay NO attention to whether you can see the sights or not, just shouldering comfort. Mark that spot on the stock the same way that you measured the proper drop to see the sights. Those 2 sets of numbers will tell you what drop you need, and most importantly, WHERE you need that drop to be.
Then there's the length of pull. It varies depending on grip and wrist style. The more vertical the trigger hand grip, the shorter the LOP. Not a lot, but between 1/8" and 1/4" depending on your hand size and musculature. Make yourself a "try gun" out of plywood with a grip similar to the style you're going to use, and shouldered in the location it will be shot from, with the type of clothing you expect to wear. Use a nail or something to represent the trigger. Keep adjusting the length (shorter or longer until you feel comfortable with your hand position for your LOP. Now you have the 3 important elements you need for your build. Odds are pretty good that the factory stuff and pre-carves are pretty close, but they will not be perfect.
The last number you need is that for your cast-off, That's the distance from your cheek weld and cheek bone to the center of your eye socket. Fat faces need more cast-off than skinny faces. For most people it's between 1/16" and 1/4" at the location of the cheek weld, again based upon facial structure. Now you know exactly where to have the cast-off located. Comb shape has an influence too. A flat comb will fit differently than a rounded one.
Now go do your build.
Modern guns have a relatively horizontal (and sometimes flat) comb so that variations in placement of the cheek weld don't matter that much. If you crawl forward on the stock, or sit back, the drop at the comb is about the same. With traditional ML'ers the comb isn't parallel to the sights (it slants), so most people wind up having to "make due" with a build that does not entirely fit them, and their scores suffer as a result. You shoot the best when you see the best, and are most comfortable in position, which means you aren't straining.
Sorry for the rant, but a proper fitting rifle is important as a proper fitting pair of shoes.
