It’s much easier to just fly for 5 mins, land, check the battery voltage and capacity, then fly another 2 mins, rinse and repeat until you get to about 3.6v per cell and around 30% capacity remaining. That’s your total flight time, it allows a margin of error so you don’t land with no power to spare or puff your pack. Set the timer on your TX to do the right time to give you the above voltages when you finish, then it beeps when time is up, so you can land.
Most FT planes fly for 5-10 mins depending on how hard you fly and the air temp.