I can definitely see your point there, but there is no way I could possibly tell you how many ml of juice I smoked for the last 2 years daily. Or even the last few months. I have always until the last couple of weeks just used factory purchased cartrdiges, the number per day varied depending on how long my day was - how long the week, etc.
I also have not kept track of my usage in ml since I started using juice because of mixing, experimenting, etc. So I am unsure of what the best measurement for all of this would be, but I think perhaps just an aproximation of usage would be best. As in I usually have my PV in my hand and puffin so many hours per day on average - or something. I dunno, just a thought.
My .02
Take care,
- Hap