Advice on Jigging HS stabilizer skeleton

Since it's close to Halloween the topic of skeletons seems fitting. Specifically how to properly jig the horizontal stabilizer skeleton for skinning.

I am at the stage where I have completed the HS skeleton and need to jig it square and true to install the skins. I have not trimmed them from the state they were in when they left Vans. The skeleton is fastened in the jig as per the directions and I am a little worried about proper alignment before I think about a drill bit touching anything. I've read over the instructions multiple times and I understand it.

from the manual:
Reinstall the skeleton in the jig and carefully true it up. The skeleton should now be pinned firmly in place, the tip ribs centered on the upright line and perpendicular to the rear spar, the center of the front spar held on the spanwise centerline, and all the spars straight-no bows or twists.

My dilemna is how to ascertain that everything is centered on the upright centerline. Seems inaccurate to use a level since nothing can be used to hold it squarely against. I could draw vertical centerlines on the tip ribs but I can't say those would be entirely accurate either. What have other builders done? I've considered stringing a taught line above everything on the verticle centerline from which I could hang a plumb bob. I do have a self leveling laser but honestly I don't trust it much, seems off vertical to me. any suggestions folks?

How much chance of introducing twist (washout/in) is there? The skins seem like they align the ribs really. The threaded rod attaching them to the jig uprights holds them well at 90deg but not so well from twisting.

a) Make a vertical post at each end of the jig.

b) Either from a centerline on the base plate of the jig or better, from a laser through the holes of the hinges, locate a mark near the bottom of each of these posts. I had small "buttons"made that fit snugly into the hinge holes and have about a .030 hole". This was small enough that I was able to verify the straightness of the hinge line on the jig.

c) Run a vertical line (a plumb bob is handy here) up the sides of the posts from these marks.

d) Drive a screw in each post so the screw side is tangent with the line (the screw hole center is offset from the line by the radius of the screw) on each side.

e) Carry a taut line (nothing to teach it, just make it tight) from screw to screw. Now you have a reference above the center of the horizontal tail.

f) You can hang a plumb line from the taut line and check the inboard and outboard ribs for twist. I used the center of the rib as the reference, and later, with the skins hung on, the center of the skin at the inboard and outboard ends.

Dave

Thanks David, not shown in the picture are the vertical uprights of the jig that run from floor to ceiling. The taut (haha) wire run on the centreline /plumbbob is a good method and I trust gravity more than my laser level.