I'm aware of all that you say. However as long as you load the cells enough to make the cells deliver some current, the voltage reading is a reasonable indication of charge level. The difference between my readings and yours will simply reflect the cell and wiring resistance.

If you actually see the reading drop over fifteen seconds, then the battery is pretty much discharged. The no-fly reading is one flight-time before this. I ran the discharge curves and know what voltage this is according to my ESV.

My Raptor can remove about 780 mAh in 45 minutes of flying - this makes the average current about 1 A. I measured this by seeing how much further I have to go to completely discharge the battery (usually about 600 mAh), then subtract from its measured capacity (1380 mAh).