The maximum mod of 189 gives a minor 6th (7 semitones).
So in theory (haven't tried it),
for each semitone you want to add, you'd up the mod level by 27.
And to go above that m6th, use a 2nd mod slot, like Examigan says.
Kind of a PITA, but it's a pretty old synth, probably made before semitones were invented.
