Exactly. Doing all this math when the main estimator says X is exhausting. Obviously I use percentage now but there is a problem with that estimate and a lot of people just want to pretend everything is fine.
Include weather, recent trips, lifetime consumption rates, type of road currently driving, etc.
Do anything other than the absolute simplest, laziest, most optimistic estimation which is "hey I bet you'll get the almost impossible-to-meet spec consumption rate!".