LOL, but I like math! And I'm not even a math wiz/engineer, but again, these computations are too hard for a simple minded human.
So, from my little 7 min test, I found that after:
1 min, I used .009103 gal
2 min, was about .017619 gal
3 min, .026636
5 min, .047058
7 min, .062842 gal...