I have always used a timer the one from Radio Shack its loud and easy to use.

for those who run there planes out of fuel and deadstick it in thats not a option for flying scale planes, if your flying a 1/2A floater or a big wing trainer then sure, but a mustang that would be a no.

for the OP just run a tank of fuel out and time your run then set the timer for a couple of min short of that time its easy.