What for you need a calculator? The ratio of the diameter of the two pulleys will be the same ratio as the rpm ratios.
Diameter 1 * RPM1 = Diameter 2 * RPM2
Example:
You have a 24" flywheel turning 650 RPM and you need 1800 RPM at the generator. What size pulley do you need on the genny?
Diameter 2 = (Diameter 1 * RPM1)/RPM2
Diameter 2 = (24" * 650 RPM)/1800 RPM
Diameter 2 = 8.67" pulley will give you 1800 RPM at the generator.