[brucepick]

I looked at a bunch of oem-looking spoilers, in the large parking lot at work today. They all seemed to follow my idea in post above:

I could sight from a point behind the car, where I would be in line with the line of the roof just prior to the rear glass. Depending on the car, I had to be 10-20 feet behind it, crouching down. And I found the rear edge of the spoiler meeting that sight line.

One exception was a Porsche that had a double wing, like a biplane, with about 2" gap between them. The lower wing was positioned as I described, the upper one wasn't.