I've just sent Markbass a message on their official site asking about their ratio knob increments. I'm also curious about the values for their attack and release knobs, so I asked them about those too!
For more of the jazz bass tone to come through, I'd recommend turning down the gain and the threshold up a little bit -- this will reduce the proportion of your signal that is compressed, so more of the natural growl will come through. Lowering release just reduces how long the signal is compressed for (not the actual amount/proportion of compression).
I tend to prefer more subtle/transparent compression, so I usually run with something along the lines of:
Gain - 11 o'clock
Threshold - 1 to 2 o'clock
Ratio - 9 o'clock
Attack - 10 o'clock
Release - 11 o'clock
Volume - unity
For slap/staccato/percussive play, I'd turn up the attack time and turn down the release time a little. This seems to work for me, but like you, I'm not sure what actual values these positions correspond to.
I'll let you know if/when Markbass gets back to me!