There is no ideal height as stated by a manufacturer. They have (reluctantly) given out a spec figure for humbuckers as people want it - mainly as no amount of 'adjust by ear' seemed to satisfy. If there is one given for p900's (as a start point for adjustment by ear mind) on the site somewhere, its probably about the same as a humbucker.....maybe a bit more gap as p90's that I have like more screw height above the bobbin in general. I have 5 real 50's p90's and they're all at different heights.
i usually like my neck a little low with the poles raised above the cover just a touch and the bridge set a little high with the poles flush or just lower than flush with the top of the cover