Well this was a case of 1 hr of work and 1 hole drilled. Not exactly massive progress, but at least I am taking my time and making sure it is right - this is no place to rush. I located the 3 bolt holes on the bottom of the rear spar of VS (vertical stabilizer) using my handly Craftsman laser level - see pics. Works good and sure beats trying to measure inside the channel where VS mounts. I traced the line with a straight edge and marker where the laser line was.
Used the laser to locate the bottom bolt hole location.
Here's the vertical axis defined by the laser that allowed the bolt hole to be located in the center.
