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I'm not aware of any mathematical formulas Art.  Maybe you'll come up with one that you can share with us once you've got this all down pat?

It doesn't take hardly any time at all to mix more up on the job site if needed.  I prefer to mix some more up, vs have lots of left over at the end of the day.  But that's just me. Others may suggest otherwise.

Jo shared some other good info with you.  Have your ground tech watch those collection bags closely too.  Especially when you're just starting out.

I read something on the forum months ago, I wish I could remember who wrote it (Chris?); Take your time, especially as you're just starting out.  Having people drive by and see your business in action is a good thing - make the most of it.    Allocate most of a day to your first job if needed, take your time, and grin ear to ear looking at that roof once you're done.