Unhook the muffler pressure line and when the tank is full the line will start sputtering before a stream of fuel start flowing. Stop pumping when you hear the sputter, you'll be within a 1/4 ounce or so of full.

With a .40, figure you have two minutes per ounce (a bit less if you fly wide open all the time), so with an 8 ounce tank you should get about 16 minutes before you're empty. To be safe, think about landing at the 12-15 minute mark. If you don't keep good time in your head, figure you should increase your height the longer the flight time goes so if you do run out of fuel you'll have plenty of time to set up a glide to landing.

Don