I think QL calls it "start pressure." Just a fudge factor that is added, and it is all a guess (your guess). Cast bullets have very little engraving pressure regardless of design.
Sorry, my bootleg QL died and I never replaced it, so I cannot do the calcs for you. I had some QL data posted at one time but it may have been lost in the server move. At one time, John Ross had some 700 grain pressure tested data on his site. http://www.john-ross.net/