OFDM / FSK channel spacing guard

What’s the spacing between a 20MHz OFDM and a FSK?

I’m trying to find it out:
According to the manual (PMP 400-430 and PTP 200 User Guide Issue 5, page 32), the spacing or channel guard band between an OFDM 400 series at 10Mhz, and a FSK at 20Mhz is:
25 MHz.
The formula for this seems to be: (BW1 + BW2) / 2 + 10

So for 20Mhz it shoud be:
(20+20/2)+10 = 30 MHz spacing

Am I missing some better doc or KB?
Is the calculation right?