Ahhhh......you do not really need that for a simple discharge unit. I use a 24v/35w halogen bulb to discharge the packs. Gives a nice gentle discharge down to cut off without stressing the cells.
Anyhoo.....to answer your question on rough and ready calculation on current draw at hover:
If we take a 5700mAh pack and fly it down to 20% we would get roughly 15mins (give or take).
So 4,560mA consumed in 15mins therefore we can assume that approximate current at hover is a little over 18Amps (around 18.2A). Obviously considerably more at climb out etc.
So taking nominal pack voltage at 3.7 per cell x 6 gives 22.2v that equates to roughly 400watts.
So my 35watt truck bulb is nice and gentle.
There is an auto low voltage cut off (LVC) to stop the cells going below 3.1v and causing damage.
Generally anything around 3.3v is sufficiently drained to accommodate pack calibration but the pack will switch itself off before any damage is done.
You then simply need to allow it to recover back to ambient (usually around 30mins) and then fully charge to 100%.