It's a massive can of worms. There was a diagram floating around on many S30Z based forums that shows mirror positioning etc.. and many poor souls have perhaps followed it incorrectly and drilled in the wrong spot as a result.
My car HS30 00051 still has the original factory guards and fender mirror holes in it. Which I've been meaning to measure up for some time now, but don't have direct access to the car atm.
But I believe and @HS30-H will no doubt correct me if I'm wrong, that they were offset from factory in terms of position from driver/passenger and ultimately where you put them is really up to you and where you think they look best. More so than strictly following a diagram to the exact mm.
Also measuring from a rounded edge of a guard (outer guard) to an exact location is difficult at best.
I'd offer to measure up my guards on HS30-00051 but the car is not stored with me at the moment. So will be some time before I could do so.