[sfilmeyer]

I get the point you're making, but I can't resist pointing out that if you start with 0 and increase regularly by 20%, you'll still always be at 0.

[kentlyons]

Yep - obviously corner cases are left up to the reader! The other way to think about this is to work backwards from your guess of a final price. So that would give you the size of each step. Likewise 20% is a pretty random number and can also be tuned. The key insight is the data and math should drive the decision.