If the motor spins 800rpm at 72v then ÷ the 800rpm by 72v to get the kv per volt and then × it by your rated voltage not the cells max volts and you get your expected rpm, It should be 533rpm.
The working current should not change whatever the motor is rated for use, There should be a max torque output for the motor if you apply the rated current you should still have max torque at zero rpm but it will fall away faster than using higher volts.
Last post I mentioned motor characteristics with points on a graph, that will alter depending on the voltage and current applied so the main thing is to make sure you always have plenty of start up torque in reserve so you can climb from a stand still and have decent efficency.
If your wheel spins at 80kph and your ride speed is 50kph when loaded then there's a high demand on torque and efficency will be crap you need to look at the graph to get geared decent and with a midrive it's easy with a dd hub it can be more tricky but they can still be torquey enough when in small rims.
This graph shows how the torque and efficency changes as the volts and amps are both increased, (a mildly overvolted motor)
See how there's more torque available but the efficency zone has gone further up the scale and there's more overall power usage so with out careful gear selection it could lead to a motor that is using peak power constantly again a bad situation.
It's also possible to see from the graph that overvoltIng the motor and gearing it lightly so it can spin to max rpm easily can give an efficency of 75% at 250rpm and still be producing torque with the higher rpm over the stock motor that was all out of torque at that point.
The overall power will still be usable if the torque demand is low enough for the motor to have no issues in reaching the rpm even when climbing it's efficency will be higher than stock and the gearing back a bit and overvoltIng can actual give better climbing torque with simular speeds but much better range.
Running low volts is not a problem it can actual help as efficency drops down the chart a bit I find with my dd hub i run it at 10s so the rpm I need to reach for efficiency is lower at 20mph but nit as good as higher volts, on the plus side I get little wind resistance at 20mph so I've made this my sweet spot with gearing of the hub in the wheel and my self peddling along, I can get upto 30 miles off 350wh so lowering the volts to the hub lowered my efficency a bit but being a hub Its hard to gear down the only option was to use the smallest rim i could lower the volts to make efficency in the small wheel at lower speeds and then the lack of wind helped me gain overall with a steady 20mph easily achievable, where a geared down mid drive could take that 350wh past 50 miles at a slower pace and geared lighter but it needs gears and to put it's power down through the chain to do it, I'm well happy with my ride no gears just simplistic and robust all over.
