there was a discussion on how battery capacity should be chosen in another posting today. I therefore sat down and gave the problem some thoughts. After some time I had a solution and wrote the results down in a short report (see attachment battery_capacity.pdf ).
For those of you, not interested in reading the whole report, I found a very short and simple way to choose the battery capacity to optimize flight time. The calculations show that the battery should be twice as heavy as the airframe without batteries (but all other things installed).
I guess this rule gets as simple as it can :D
p.d. I added a new version of the report (just use link above)