I'm about to make your next job a little easier.
I came across this handy little gem this past weekend when I was asked for a quote for 1000 duplexed business cards. For me, math in any form is daunting, so I quickly Googled "paper cutting calculator" which led me to this:
It calculates waste percentage, factors in your gripper area, and you can even select long or short grain.
Once again, the internet redeems itself to me.
Happy cutting, Ladies!