That's how I do mine Iain, then depending on miles driven against the no litres it took to re-fill.... As I keep a log for business, I have calculations cover MPG over 20k miles.
I only mentioned, as a mate of mine was doing the same thing, but started off thinking the tank capacity was as per the book, and made the school boy error of thinking it took the same to refill each time ... doh!!! (Mind you, I should add, he drives until the red light is on and the car is on fumes) before filling up. Have told him that isnt good for the car, as all sorts of gung gets chruned up that way (ie. when the tank is nearly empty).
Edited by: K.L.Richards