Model Market Fit is the concept that your market (and # of customers within your market) influence your model.

First, define your target market. You should have done this already at the Market Product Fit stage anyway.

After you have defined the market, you can do some qualitative research to understand what their willingness to pay is for your solution to build your ARPU hypothesis.

for SaaS businesses where there aren't strong network effects, I use 10% as a rule of thumb. Great SaaS companies will capture more than 10% of their target market over time, but this is a good starting point.

If you've read some of my other material I also typically recommend starting with a niche market in the earliest days and then expanding out.