I looking to set up a reduction drive for a 40 HP rotax using a 50" Hascon multi fan prop. Rated at 450 RPM. Would someone know the correct Pulley ratio and sizes for this configuration. Will be using a 5V belt.

I came up with drive pulley 4.3 and driven pulley 15.9". Would like to get the 15.9 down some ware around a 12".

Thanks Gene