Scolpit,
I do exactly what you brought up. I have a countdown timer that I use when flying. What I do is take each plane and run it on the ground at full throttle so that I know how long a tank of fuel will last. Then I use that time as a basis for how long I can fly. I like using the timer because I don't have to take my eyes off of the plane to see how long I've been in the air. When it's time to land the timer beeps to let me know. I've been doing it this way for 9 years now and it's always served me well.

Hope this helps

Ken