Here's what I do for this problem. After the motor is broken in, I will run the engine on the ground and run it at full throttle and I will time it takes for the engine to die. I will run 3-4 tanks through it so I can get a good idea of how long I can fly before the engine will die. I will then subtract a minute or so off of that figure. I will then set my timer for the time I came up with. I've never ran out of fuel doing this, and I've always have plenty of time to get the plane down after the timer goes off.

Hope this helps

Ken