In a similar way as locating the center of the longeron for the front spar of the horizontal stabilizer, you can measure the same distance in the back of the fuselage at the F-712 bulkhead inorder to determine the position to drill the AN3 bolts that hold the elevator stop to the longerons. Quite a bit of load goes on the rear spar of the vertical stabilo so edge distance is important here.
At first, I had already clamped some alignment bars to the F-712 in order to line up the F-712D elevator stop and drilled the 3 #30 holes for the rivets.

First drilled to pilot holes #40

And enlarged 30, 13 and reamed to #12 for the AN3 bolt.

Blurry image but clear enough to show the edge distance on the longerons is perfect.

All done, ready to mount the vertical stabilo now.