Hey Ian,
Well on the same day a few months back, I checked the digital meter in two locations and used the dipstick. In my garage, since I' ve owned the car (Boxster 2.5 tip last July) its always shown 1 bar above max, then I drove to a garage forecourt it showed 2 bars below max line, and later using the dipstick in my garage, the oil level was just below the max line. I think of all places to use the digital meter, the garage forecourt should be the most reliable, however a few months back one led to a display of 1 bar below minimum on my meter!
I' m sure you' re right that the pitch and tilt of the car will probably also affect the dipstick reading, but probably to a lesser degree, not sure tho!