I haven't fully played with it yet, and the numbers are going to be different for each model of pedal, but start by setting X to 70 and Y to 20 and see where that gets you. X should be between 50 and 80 and Y should be between 20 and 30 for any normal volume pedal. There should be an ideal set of numbers that correct each pedal, but we're going to have to figure out what they are.
Geek stuff: Volume pedals work differently than expression pedals - expression pedals have a linear taper. When the pedal is at 50% position, the value it sends to the GT is 50% of maximum. The pedal position = pedal value no matter where you are on the pedal. Volume pedals have an approximation of log taper. The first 50 to 70% of the pedal's travel has the value increasing slowly, and the last part increases much more quickly. X=70,Y=20 means that the slow part of the taper is in the first 70% of the pedal's travel. At 70% of travel, the output is only 20% of maximum. The remaining 30% of travel goes from 20% to 100%.