You'll need to look at this as a two headed problem, the drop from the service to the pump house and the drop from the pump house to the pump.
I'll assume the pump guy is providing the wiring and connection from the the ground down to the pump. He has a cable his plans to use. Figure the voltage drop for that part.
To keep within the recommend 5% drop, see what wire size you'll need to have for the feeder you're providing.
Running a pump on lower than designed voltage will cause it to run hotter and thus fail quicker.
