ORIGINAL: bkdavy
You're not aligning the wing with the fuselage. To do this, you should use a piece of cotton string or a stiff wire. Put a pin into the fuselage at the centerline of the tail. then use that to set the distance to the rear corner of the wing. Adjust the wing as necessary until the distance is the same on both sides.
This is how you determine that the wing is square in the fuselage. Do as Bkdavy said, measure the distance from the centerpoint of the tail to the wing tips and adjust until they measurement is the same.
Ken