RE: Comp ARF Lightning + 7xDS8911 servos = VERY HIGH BATTERY CONSUMPTION
The average cord is at the center of gravity of the flying surface ( assuming that the build density is even ). On a rectangular shape it is at the middle. Not on a trapezoidal shape.
Of course you cannot remove every FC to check its CG. Just evaluate it roughly and that should do the trick.
On your picture, the flaps average cord is more or less at the middle and the aileron at the first 40% of the span.
You are correct for the FC span: it is the length of the control.
I need the control horn arm length. The servo arm is given as an output data...