>It's better to start from how many times you want to go to the gym
Which you may or may not get close to.
It's easier with subscriptions generally. It's a good idea to constantly be reviewing how much you're using the subscription and how much value/benefit you're getting from it. And of course, that equation can change over time if the subscription fee goes up or you stop using something as much.
This is also dangerously close to going backwards about it. Figuring out how much you need to do something purely to "get your money's worth" out of it is the sunk cost fallacy!
It's better to start from how many times you want to go to the gym, multiply it with your desired cost per visit, and then see if this total exceeds the cost of the membership.
I know that's what you were suggesting, but the way you presented it seems to me, well, backwards!