your formula looks good, but its a bit too simple. In your formula, you assume a flat rate of cancellation, ie, 50% of the previous months rebills cancel. The reality is its more of a curve, so I would say ~65% rebill once, then 35% rebill twice, then only 10% let it go for a 3rd rebill. After 3 rebills, you tend to either get 0% rebilling or you get the occasional "lifer" that will rebill for ages.

These aren't accurate rates, but its just to demonstrate what I am suggesting.

cheers,
Luke