Here's a call for at least empirical research on this subject....
How many ml's of juice per 100 mAh? Might that not be a "better" average? I think it might be more meaningful. After all, what IS 1 hour of vaping? How many seconds per draw, how many draws per session... where does that hour even come from?
But if there's a ballpark estimate - and just pulling numbers out of the air, here! - of, say, 100 mAh per 1/4 ml, then a vaper could look at his or her topper, say a 5 ml carto tank, and think "hmmm, I'm going to need 2000 mAh of battery to vape this. Better take two..."
Tell them to get an eVic or an iTaste with a puff counter thingy.