Mike....re: centre-spotting the diag. From what I've read, some people work out the off-set needed for the diag. & then spot it at that measurement....ie if a 2mm off-set is needed, put the mark on the mirror 2mm off-centre.
I wont be bothering to do this though. I think I can guess a 2mm off-set of the diagonal using me lil' peep-hole-do-hickey.
