Lee,
It got up and left years ago
When I was in my youth (yes I can actually remember that far back), I had a MK1 Escort RS Turbo. At the time, I though it would be a good idea to stick on the bee sting aerial from a MK2 Golf GTI 16V (it was all the rage at the time.)
Measured carefully, made a cross on the drill position with masking tape, and started to drill with a pilot drill. Went though no problem, but also went thorugh the headlining and the foam noise insulation that was stuck to the roof also nearly caught fire and started smouldering. Sorted that out and went the the full size bit. Firstly it slipped and the drill left a lovely mark across the roof. When I finished swearing, and fitted the aerial and stood back to admire my work, I noticed that I hadn't measured properly and the aerial was neatly 1½" across one side of the centreline.
Now do you see why I won't trust myself Lee