Well, to me, following the diagram did not put them in a position I was happy with. Also, the celica stalk and mounting plate is very thin, so it wouldn't have made a big difference if I measured to the edge of the mount rather than the centre, if that's what you're getting at...
Another position I found in a thread somewhere on here I much preferred so I went with that. Put them wherever you want, doesn't really matter as long as you are happy, I'm just saying that it's possible to get really good visibility out of them.