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.
Surely using bitcoin as currency is the primary factor to give to bitcoin a strong velocity.. what mean more price stability etc..
but there the bitcoin network capability of holding such numerous transactions is actually very far from to be possible.
This in first was due to egoistic attitudes of some actors at a point this would be legitimate to suspect these were in reality voluntary acts of empoisonning/sabotage ...
look at Bitcoin cash for example, it's capability of holding transactions... and for ridiculous fees - in addition that we can't say that that is making problem!
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.
You're talking about legislations that made very complicated fiscal procedures to the users of crypto-currencies, right?
PS: Excuse if my english is not very fluid.. It take me time to write in english, and when I am a little tired like right now I don't have at all the strength to remake the texte to clearer! Thnx