I'll just quote the manual:
There are a limited number of valid numbers that can be used: 3 (3.58MHz), 4, 8, 10, 12, 16, 20, 24, 25, 32, 33, 40, 48, 64. These are the only frequencies for which PBP is able to accurately calibrate its timing. If you use a system clock that runs at a frequency that isn't listed here, your timing will be scaled when the program executes.
That's pretty clear to me.

/Henrik.