Chuck-roaniecowpony
TVWBB Super Fan
I was thinking similarly. I weighed one brisket after trimming and cooking and it was 45% of the package weight.This illustrates why I avoid the temptation of offering a recommendation on these questions. With my own experience as a novice I'd suggest factoring a 40% loss from the initial weight. It's nice that this site has members whom are well-experienced
But I would also add that in my humble opinion one can overshoot a PP calculation without risk by freezing and serving it for future meals.
Best of luck, and enjoy your party @Michael Smotch!