You have to find the equilibrium point of two variables: as BTC's purchase power goes up, the number of people who will transact in it will in fact decrease, not increase, because it will become too valuable to spend. To have an idea of what the intersection is, probably you want to find out how many high volume (let's say the equivalent of $50,000 in purchase power, or 500k, or 5 M, it's arbitrary) transactions happen every day and how many users that is.
Discussion
Another thing to take into account though, which I think makes BTC potential to he spent even lower, is that those spending it will not do it unless they have a reasonable expectation to be paid in BTC as well. So, if I transfer 500k in BTC to pay for a house, I would only do it if I have another asset to sell that I know I will be able to be paid for in BTC. Unless I'm simply liquidating my wealth because I'm bankrupt, or I'm old and don't intend to pass it on, or something.