Hey Mike!
The actual utilization is going to depend on a lot of factors, some of which you won't be able to predict in advance.
It is affected by how much you charge, what you offer, geography, and the kinds of needs and behaviors of the people who end up signing up for it.
I've found that people largely use what they pay for, with a small variance—so you might plan for 80%-90% utilization.
If you want to be on the safe side, go 90-100%.
Even on the aggressive side, I wouldn't advise projecting lower than 75-80%.
Better to play it safe and know the model would work even if mostly everybody used mostly all of their membership credits!
Tony
PS - I have a space capacity calculator and am testing out a new service to help people plan their spaces. Let me know off-list if you want to give it a spin!