I've always assumed that you measure it back to the same spot as the driver's side mirror. I'll be interested to see if anyone posts anything different as this is an upgrade I would like to do soon myself.
I also plan on doing this myself, and was just talking about it today at the "Nolookingback day in the park", the only mistake I have heard of people making is ensuring that the mirror does not interfere with the vent window.
I made a template and mailed it to 68FB390 a few months ago. PM him and maybe he will pass it on to you guys. If you don't have any luck contacting him let me know and I can make another from the copy I kept for myself.