For a starting point I tend to fret on last fret, put and put one of my 2mm picks in there. From there I lower (and get back up to this point again) to until I find my favorite spot. That spot usually is 1-2mm away from where I started from, so quite a lot, but I like to try a lot of different hights, although I never have them close to the strings, to get an idea how this perticular PU will react to the changes, which helps me eventually finding the sweet spot.