I make all my measurements for wing and horizontal stab placement from the center of the fuse at the firewall. If I should be aff a bit on wing installation, I don't want to carry that over to the stab.

As Minn mentioned, one eye works best (for me) when trying to get the vertical stab 90 degrees to the horizontal. It also works very well for making sure the horiz stab is level with the wing.
Dennis-