Most melodies are made up from a few common scales, but sometimes you may come up with things that are outside any known scale (such melodies are usually the interesting ones, in my opinion). If you want to make Intelligent Pitch Shift voices to such melodies or riffs the Custom Scale might be useful. With it you can choose almost any combination of notes that will fit that particular riff or melody, even if it would be useless for anything else.
Creating a custom scale
Write down the notes of the harmony melody you want to play, lets say the following pairs:
F+C#, E+C, C#+G#, D+A, C+G.
Program all the notes used above into the G-Force as a Custom scale. The above notes would result in the following scale: C C# D E F G G# A.
Count the intervals between the notes in the riffs. In this case all the intervals can be set to -4 in the Voice parameter. C# will then result in an F, C in an E and so on (like in step 1 above).
Complicated scales
What do you do if you want to change the interval or Voice in the riff, for example if you want first an F+C# pair, as in the riff above, but after that an E+C#? There are no ordinary scales where two notes can share the same interval (in this case C#) but here are some things one can try:
Set each Pitch Shift Voice to a different interval and switch between the two Voices with an expression pedal (by turning up/down their respective Voice levels).
Make different Pitch Shift presets and switch between these while playing the riff. But note that the Custom Scale is global, so the only thing you can change in presets besides the Voice intervals is the root Key.