What I usually do is fly in increasing increments of time and check the charge left in the battery afterward. Generally start off with a 5 minute flight, check the packs, swap to another set of batts and fly for 7 or 8 minutes depending on where the first set were at, the goal being to find out how long you can stay in flight before hitting the 80% discharge. Once I know that I set the TX time for a minute or two less than what it takes to get to 80% discharge so I have time to return to the takeoff point and land with a bit of a safety margin. Occasionaly I find the multi a good distance away or fighting the wind trying to return to takeoff so it's nice to know I'm still within battery capacity as the timer goes off and I start the landing process. Also a good idea to set the voltage warnings maybe a bit below this point so you have a secondary warning if you stretch the flight time out a bit.
Ken
