A rule of thumb I use is:
REQUIRED (lbs) +10% + 5% then half that amount.
I.e. 5000 lbs + 500 lbs + 250 lbs = 5750
Then 5750 / 2 = 2875 L.
I find working in these few steps quick and easy for mental calculations and gets me within he ballpark for the amounts of fuel I take. YMMV.