Initial To Rebill Ratios On Individual Paysites?

I was wondering what you folks who owned your own paysites aimed for in terms of initial sales to rebill volumes when it came to members joining your paysites?

Today ive been going through some stats on CCBill and it would appear that in terms of 'revenue' the top converting Condom Cash sites have over 75% of their revenues generated through continual rebills.

The weird thing is, im also seeing some of my private sites, which have the exact same amounts of content inside them, including some which actually have more than my public paysites, dont appear to be rebilling as well.

So i have 2 questions...

1) What sort of initial sales to rebill percentages do you aim for on membership revenues and;

2) What would cause 2 same niche sites, with the exact same content (sometimes more) to have differing initial to rebill ratios?

Any thoughts/feedback on this?

Regards,

Lee