looks like you’ve got a decent handle on the math.
3x wash of the same biomass with enough ethanol to cover the biomass (~4l/kg) should give near complete extraction.
if you wanted to be conservative you might use 95% extraction efficiency, but you really should be approaching 99% with 3 washes.
how did you decarb?
while it is true that you “distilled” the ethanol off your concentrate, calling your concentrate “distillate” is incorrect. that moniker belongs to the portion of your boiling flask that was vaporized and re-condensed…which your concentrate did not do.
“distillate” around here is used specifically to refer to cannabinoid concentrates that have actually been moved from flask A to flask B via evaporation/condensation.
“solvent recovery” (or “removal” if blown to atmosphere rather than re-condensed) is the preferred description for the reduction you performed in your Source.
21g x 1000mg/g => 21,000mg biomass (units make the math…)
21000mg @21.7% THC => 4557mg of available THC (but 18% is probably a more reasonable potency unless you used all top shelf buds).
@18% THC, and 95% efficiency. I get 3591mg
with 3590mg in 720g you should be right at 5mg/g…but that isn’t the end point for dosage calculation.
you still need the weight of each gummy.
eg a 10g gummy puts you at 50mg/candy. a 2g gummy at 10mg. using molds so each gummy is approximately the same weight is important here. at least if you’re hoping for consistent dosing (I see you get that. pretend I wrote it for the next guy 
).
I suspect your bioassay based “potency test” points to incomplete decarb rather than incomplete extraction…but the only way to really know would be to get them tested by someone with a GC or HPLC.
most folks have a fairly narrow range in which they find the experience enjoyable. 50% low and many won’t/will barely notice the effects. at 2x their preferred dose, many will NOT enjoy the ride.
this was data I deliberately gathered from my dedicated bioassay team once upon a time. my “preferred” dose used to be about 100mg, it seems to have dropped to ~75mg these days.
I’ve run into folks who can get wasted for 24+hrs on 0.1mg THC, even at 20:1 CBD/THC and others that 500mg THC doesn’t phase…