This is the problem for any new service or product; what will the adoption rate be for acquiring, maintaining and managing customers throughout the customer lifecycle. There are formulas that you can find on the web that may help you, however my suggestion is regardless of the methodology used to define how you acquire the first paying customer, how you handle a loss of a customer, and how you move a customer up the value chain that you model three scenarios, worst-case, expected case, and awesome case.
I have witnessed investors lose-interest by painting the only rosy picture and by no means am I stating you should not have a explosive growth scenario if it is warranted, but I have found a lot of interest in what are you going to do if it is not a explosive. The worst case scenario should still allow for investor payout/exit and full funding of the operation. In your scenario, I would also cover customer not only moving up your value chain but also what percentage per month may go down the chain, as you state you have a freemium, basic, advance and premium. Can a customer paying month one month as a premium subscription move to an advanced, basic? Do you allow for your customers to go from pay to free, what percentage of customers will be doing that per month, how do these customer churn situation affects your subscription rates.
What are the on-going plans for moving customers up the chain, while still trying to acquire new customers per month? I will make the assumption that marketing dollars are going to be spent on each segment, acquire new customer, maintain and move up customers, and how to reacquire customers that may have left the service. Hope this helps.