You need to charge your customers in ways that they want to pay.
How Big Is Your Market?
Market Size = ARPU x Total Customers In Market x % You Think You Can Capture
The Model Market Fit Threshold & What it Means for Your Growth Strategy - Brian Balfour
Model Market Fit is the concept that your market (and # of customers within your market) influence your model. The first time I heard about the underlying concept of Model Market Fit was from Christoph Janz @ Point Nine Capital.