John:
Honestly, the best advice I can offer is to turn the knob until it sounds good! Of course, if I want an acoustic guitar sound, I pan all the way to the piezos and set my amp to it's acoustic setting. (I have a Pritchard Sword of Satori that has an acoustic voice and a little horn that kicks in with a footswitch). Otherwise I'm probably at 2/3 - 3/4 magnetic at most times. And sometimes it sounds better with all magnetic. It depends on the song, but also seems to depend on how the other people I'm playing with sound, how my tone fits in with theirs, and even the vibes on any particular night. (or maybe how my ears are working on any particular night?) The blend usually does give a nice sound. It's subtle, but adds character.
Bill, tgo