Try setting it to about 11 o clock with the resonance also around 11 o clock. That's a real sweet spot in my opinion. Also it's extremely useful when mixing the p6 with different instruments. The p6 is so warm and fat that it will take over a mix if you don't use the HPF, especially if you're playing chords. Sometimes you want that, but if you're playing with other instruments then a lot of the time you'll want to tame the sound to let the bass player be heard, or for the snare drum to sit right, or for your other P6 tracks to not overpower your mix.
It's also useful for emulating the bass cut that a ladder filter has implicitly. For example, this patch wouldn't have been possible without the HPF ->
https://youtu.be/ETZB9bmMqHM?t=23Finally it works really well when you're using distortion. It cleans up some of the flabbiness you run into when distorting low frequencies, and can give you really great overtones when driven hard. Try setting the LPF to have pretty high resonance and the HPF resonance to 11 o clock, give it some distortion and play with the filter cutoffs until you find something you like.