DUDE your Math is all screwed up:wallbash::wallbash:There is a rather simple formula.
Divide the set. Let us say it's 3,5,7. That set equals 15. The 3 inch part equals 20% of the total. The five inch portion will be 33% of the total. The 7 inch will comprise 46% of the total. So if the job is 2000 feet, you'll need to use those numbers. Just multiply 2000 by each percentage and you'll be dead on.