LBH
Make the minimum 100 and the maximum zero and it will be in the correct intuitive direction
I assigned the modulation wheel to the swell pedal, with min at 100% and max at 0%, so that the mod wheel, when it is normally all the way down, makes the volume of the swell all the way up. To lower the volume, I push the mod wheel up.
The visual is off but perhaps thats the way the organ originally worked.