So without doing "wonder" pictures given the bad lighting conditions and reflections and the fact that car is in the garage, I made a few shots and measured. This is what I gathered;
1) The original M badge itself is 5 cm in length and just under 2 cm high
2) The distance between the M (beginning measurement from the right side and going left towards the wheel well and up to where the paint ends) is 19 cm
3) The distance of the badge to the black side vents / trims is 5 cm regardless of whether is measured diagonally or horizontally.
Please see the following pics in order to (hopefully) better understand the above points. Click on the picture - there are several ones and on a couple of them I have drawn it out (or tried at least!).
View post on imgur.com
I would conclude it follows the body lines to some extent, but it is not 100% straight or level with ground. Your idea of using a template is great as in that way you will most probably match it better. I think also looking at pictures and having them color printed and next to you during install is also a valuable help.
