I vape between 6 and 3 ml a day. That has been pretty consistent since I took up vaping. I smoked between 1 to 2 packs a day for 40 years. When I started vaping in August '10 I continued to smoke for about 3 weeks or so but my cigs went down to about 1 pack every 3 days. I went away that labor day weekend and made the decision not take any smokes with me. I made it through the weekend and never smoked again.
I might try a small bottle of the 12mg again and see if that will cut down my vaping. I do find that I can go longer without vaping than I was able to without a cig though.
I have no way to be certain, but I suspect that I am less addicted to the nicotine than I was at the start of the year, and since all other things are equal in the equation... that I am just reaching for a hit less often!

I do know that if I use juice at a lower nicotine level that I definitely go through it much faster, and have to vape for much longer before I feel done... perhaps if the 12mg is too harsh try mixing a little of the 12 and the 6?
I smoked 1+ packs a day for 25 years before I started vaping... from 15 to 40... and my entire working life (started when I started working)... so I weaned myself off of them slowly with putting limits on how many I would allow myself each day, and unlimited vaping. I just gradually lowered the limit... the last 1 or 2 per day were the hardest to give up.
I just find it strange that most people seem to vape greater quantities over time (although some do lower their nic levels as you have obviously been able to do), while I seem to be moving the opposite direction of same nic level but less liquid.