I have a 200 Amp Electrical Service and I would like to install a 60 Amp Sub Panel in a building 300 feet away. I will have 120 volt and 240 volts loads at the building. To calculate the size of the feeder to the building, do I use 120 volts or 240 volts in my voltage drop calculations?

Either, as both need to be within no more than 5% less of nominal at the end of the circuit fully loaded.