Maybe I'm a bit off but my Smoktech eGoDCT claims to hold 3.5ml , and I fill it 3 times a day on average, and puff on 2 boges throughout the day as well. So I might actually be closer to 15 mls.
I hope its much less. Does filler sponge up & dry up some of the juice, where I may be vaping less than I think?
What Nic strength are you vaping. I vape 24mg and go through about 10ml a day. I was about on par with you when I was vaping 18mg nic.
Also, I've heard really good things about the protank. PBusardo gives it a thumbs up.