I dont know if your just curious or want to get a formula for a reason, or just studying for a maths degree. If its just to see how well your site retains, i would be inclined just to look at the average number of months your members stay for. The work you do on the site will then see an increase or decrease in this, telling you everything.

If an affiliate asks about average retention, all they need to know is the average number of months members stay for.

Ive probably missed the point, this thread confuses me.. too many numbers