Here's spud69 explaining the same requirements:
http://www.haynes.co.uk/forums/showt...6722#post64840
BTW, on my roadster the height difference between the rear wing and the scuttle is ~8 cm, and the distance is ~120 cm. Giving the angle of visibility 3.8° below horizon if the repeater is at the top of the scuttle, less than that if mouned lower down on the sides of the scuttle. 50% of the apparent surface must be visible in order to call it "visible".