while the simplified rule of thumb from Geckodrive does not tell the whole story it gives you a starting point

Different motor manufactures give the maximum temperature as either 70 or 80 C

The basic stepper motor data sheets don't specify the iron losses so you can't be sure of how much heat will be due to eddy current losses

The need for the use of a power supply thats several times the motor voltage is all due to need to force the current to build up to the required current in the time available between steps

Since the voltage and current is not in phase
the dissipation is less than the motor current x supply voltage

John