I've never used a bought print before, I thought they came with a mat list.
Can't speak for anyone else, but it isn't worth the time trying to figure an exact, too many variables and possible errors to crop up, (pour moi)
wrong tool setup, milling green wood that shrinks prior to assembly, dropping to concrete floor, etc. I tend to figure about 15% above what I believe I need, which includes, if all goes well, rough cut dim (for acclimation then final dim) and blade width. If all goes well I have left overs for some other thingy.
Never bite the hand that looks dirty!
The more you know the more you're worth