Hi,
it's fairly easy - look for the documentation of specific HC08 derivative, especially chapter Monitor ROM. Here you find table called: Monitor Mode Signal Requirements and Options
It gives the basic idea, typically the baudrate in Monitor mode is a Busclock / 256 (some devices have a different divider, like some CAN-enabled device), you have to check.
Normally (on HC08 devices) in user mode, Busclock = External clock / 4 (where External clock is either external oscillator, external crystal, internal VCO, simply the actual clock source).
In the Monitor mode there's sometimes a second option with Busclock = External clock / 2 where one divider by 2 is bypassed depending on external pin level upon the Monitor mode entry.
To make this long answer short, the baudrate will be External clock / 1024 or External clock / 512 depending on the derivative and/or pin levels upon Monitor mode entry. That's how I calculated this 'odd' baudrate.
Hope this helps, regards, Pavel (Freescale Roznov p.R., Czech Republic )