Ah, ok.. but how do you calculate the rebill ratio and/or retention?

I have 'X' sales 2 months ago and 'Y' rebills, the next month I have 'Z' sales and 'AA' rebills, this month I have 'BB' sales and 'CC' rebills.

What is the formula? There has to be a simple calculation to put method to this madness. Where are the math majors when you need one? I'm not really concerned at the moment on how members rebill over the course of multiple months... just the basic trend.