Igalla, it all depends on what level of accuracy you are shooting for, and what type of equipment you have available to you, to get there. I'm currently working on mounting my rails and should have some concrete answers sometime in the next week or two. You can read about some of the idea's here (Layout of centerlines) on what was suggested and my final choice on how I'm going to do it.
There are many different way's to skin this cat..
I've finally got all my needed jig's laser cut and shipped here on last thursday, now I just need to finish drilling and tapping them [ I'm about 1/2 done] and then I start mounting the rails. Once I know if my route will work or not.. I'll be posting it back there on the other thread.
I guess thats really not a direct answer for you, but a start non-the-less..