dunno
I usually just do the 2-3" drop at 25' (depending on vehicle) and/or adjust to what feels (seems/looks) right while driving. If you're getting the flash (at night from non P-car owners
) then yours might be a little high.
Having said that, on the Boxster, b/c it's so low (yours especially), you can usually cheat/edge up a little higher than on most other vehicles.