1st, you need to block up the fuselage and not use the landing gear for measuring. This will provide an accurate measurement, where as the LG can flex and be slightly different in height. What IS IMPORTANT is that the main wing and the horizontal stab are parallel with each other. The plane flies on its wings. The fuselage is just there to hold the wings, as long as the wings a parallel the plane should fly properly.

2nd, with the main wing installed on the plane, you need to attach the horizontal stab (the part the moveable elevator connects to ) and insure it is parallel with the main wing.
You do this as follows:

Measure from each wingtip on the main wing to the tip of the horizontal stab. You want the same distance on each side.

Once you have the horizontal stab centered (equal distance on both sides ), from the rear of the planesight forward and make sure the horizontal stab is even with the main wing. Then you can glue/epoxy the horizontal stab in place.

What is just as important is the incidence. Incidence is the angle the main wing and horizontal stab are at. MANY planes are at 0/0 - the maing wing is at 0 degrees and the horizontal stab is at 0 degrees. Just because the plane may be an ARF or a kit you shouldnot assume the incidence is correct. If the incidence is wrong the plane can jump off the ground before it is ready to fly (too much positive incidence), can take forever to get off the ground (too much negative incidence ) and/or not fly/handle properly. If the instructions OR the the plans do not state an incidence, use 0/0. You will need an incidence meter for this. Robart and Great Planes make very good ones at a reasonable price. I prefer the GP unit over the Robart unit.