The voltage drop is independent of the circuit voltage. Wire size, length, and current are the only things that matter.The only thing the calculator should use the voltage for is calculating the percent drop.
What is the circuit in question? Is it a 120V branch ckt to a device? Is it a 240V circuit. Pick the one that is right for your application.
Disclaimer:
I've never used a voltage drop calculator. I personally would never listen to anyone that has never used one - especially if they have not used the one in question.
ice