1) If you expect a certain refund quota, lets say 20 % within 6 months, then you should already account for it: only account 80 % of the actual revenue and adjust this number after 6 months when you now the actual refund quota. This makes it more complicated but it would be the right thing to do and will help you get a somewhat decent financial plan.
2.) If your growth curve flattens out than this approach obviously does not really give you a good representation of how well you are currently doing.
2) Right, that's how I calculated 58%, but does it really make sense to calculate the base off of some arbitrarily low first-month revenue? Does it make sense that a company with strong first month sales should have a much lower growth rate than a company with abysmal first-month sales, given a certain current monthly revenue?