Aerial location measurement required on an R36 wagon please

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Hi all

Can anyone who has an R36 with the RNS510 installed at factory (and therefore who has the shark fin aerial) wagon let me know the distance between the edge (rear most) of the shark fin and the edge of the rear spoiler.

To make it obvious, the pic below should show what I mean - so if i had to place a shark fin on the car, how back from the spoiler edge should I position it? Thanks