Hello Lutz, you are right. I need to update the manual to cover the strobe speed setting.
The speed of the strobe is driven by the current deviation from the tuning target. To get the frequency of the strobe, the current deviation is multiplied with the ratio specified in the settings (defaults to 7.5 rpm per cent). Deviations larger than the given maximum are clipped at the maximum value.
Does this answer your question?