I believe that in the case of Bitcoin, the velocity should be calculated using the quantity of coins that are apparently available and released for use. I believe this because Bitcoin means a variety of things to different people and organisation, and its capabilities are fairly complex.
my first thought is to go to say that for a sustained velocity it needs a constantly new number of (new users/Interactions)..
A constant entrant of new users to bitcoin isn't just enough to create a good level of velocity of the coin. With the current number of users of bitcoin all over the globe yet the velocity doesn't correspond with the users and that's because so many are using it as a store of value while a very few are the ones using it as a means of exchange for goods and services. Unlike dollar where almost all users mainly using the currency as means of exchange for goods and services all over the world.
Another important thing that can sustain a good velocity of bitcoin is the lifting of restrictions imposed on bitcoin by some countries around the world as this restrictions has made the users in these countries focus on just seeing the digital currency as a store of value whereas the BTC has so many usages that many users haven't experienced it with.