Minimum voltage drop to run a circuit
Typically you design a circuit with parts where you have datasheets. These datasheets will explicitly tell you in which voltage range they will work and the designer of the circuit will make sure, that the parts are supplied by the correct voltage by specifing that as input voltage. If other stuff (e.g. a voltage divider or voltage regulator) is placed in front of that device, the voltage rating for the input voltage will change in a way you can easily calculate.

The manufacturer supplying the sub-part with datasheet (like transistor, IC, whatever) will as well use math and simulations to find the proper voltage range. In addition he will probably do extensive testing to check which voltages lead to failure or degradation.