As far as I have found out the measurements should be 18 and 26 mm.
Listed on the swedish M5 Registry is a detailed photo:
http://www.e34m5.se/forum/modellbeteckning.asp
I would say 8-10 mm between the M and the 5 is too much. 6mm is more like it. But when it comes to that I really dont know exactly what has been stated by BMW, but by taking measurements on a car that never had its badges stripped off I came to the conclusion that 5.5-6mm is correct. By looking on the photo you can clearly see that half of A (18mm), 9 mm, is too far apart. So 6mm is closer to the truth, I think.
18 and 26 mm are numbers that I got out of reading BMW material.
But if you look in some sales brochures you will find a Sebring grey car that does not follow these measurements. The whole thing was placed many mms further up, by BMW!