Hi Fignon,
I spent some time looking through the docs, and you are right that there is very little information on the settings for the Filter Registers (FLTU, FLTL). I put in a service request and was unable to get the equation (mapping) for filter bits to clock frequency. The default value of 0x0C0 maps to approximately 8MHz. I was told to change the value and use a scope to see the result! Basically you really don't want to use the self clocked mode (SCM) for much of anything. Move over to FLL Engaged with the Internal Clock Source (FEI), and you have much better control of your MCU clocking. You can trim the internal oscillator, and you have some choices of core clock speeds. I recommend using the Internal Clock Source and going to FEI clock mode as an option for you.
Thanks,
Rick