This seems like a proper thread to post a link to this pretty slick site I found "pun intended"

http://www.csgnetwork.com/oilfuelcalc.html
I like it because it will calculate 0.5 or 0.7 of a gallon to what ever ratio your seeking I used it to figure out how much oil I needed for 20 oz of fuel to get my desired 25:1 and no other calculate could do that so I hope someone else gets use from it like I did