You're fitting a VFD, it really doesn't matter. You could work it out from the gearing if you really had to but with a VFD your speed is what you set it to.
Here's what I did - I got a 3/4 turn pot for the speed control. I set the lowest frequency on the VFD to I think 15hz and the highest to 100Hz. I mounted the speed control pot on the machine then used a tacho to set the spindle RPM to exactly what the gear lever said it should be and orientated the pot so that the mark on the speed control knob was at 12 o'clock. So with the mark at 12 you got the standard speed and turned fully clockwise it was double.