What I don't understand is the arbitrary 250 ma draw on the ESV. (Expanded Scale Voltmeter)
The NiCd discharge curves are determined by the cell MFG using a C/5 load. I suspect that some ESV's just assume that 600mAH cells are being used and determine their finger-in-wind cutoff voltages (red zone) when their fixed load is used.
Does this mean at 5.1 volts I can figure about another hour of flight time, since the complete discharge at 300 ma took 100 minutes?
There is no way to easily determine the battery's remaining mAH from the voltage. Other trickery is needed to do that properly.