You want to measure from a point on the tip of the stab (the same point on both sides) to the tip of the wing; the distance will be equal when the stab is square to the plane. Once you get that set, you want to measure the vertical distance from the wings to the stab; they should be the same on both sides. The wings and stab should be square to the plane (measured to the center point on the tail) and need to be square with each other.

I believe this is important as I am flying a plane that is no longer 'square'. Even if you are not going to fly pattern, you don't want to learn bad habits of using ailerons to help you track through a loop because your plane isn't straight.

So again, by this point you should have measured a point on the wing tips to a center point on the plane to square the wings to the plane. Then you want to measure from the tip of the wing to a point on the tip of the stab and it should be the same on both sides (mine have all been to within 1/16" before crashes) Then when everything is square, make sure the stab is parallel to the wings by measuring the vertical distance from the wing to the stab (on a level surface, measure the distance from the wings to the surface, then from the stab to the surface; should be the same on both sides).