Don't try to be so precise with bands. I think one of the things that I have to keep myself from trying to do is equate chains or bands to an actual weight. They aren't meant to replace weights, so don't think of them in those terms. They change the resistance curve, and for the most part you really aren't going to need to use them on any kind of a max effort lift, so trying to figure out if they are going to put you above your max capacity shouldn't really come into play.

Just my two cents.