The farther away you go the more acurate you get. Find the center on the firewall and string it from there. If the cockpit gets in the way, go to the bottom of the firewall and center it there. That way the angular difference from tip to tip is smaller than measuring from the front or rear of the wing saddle. Like when you squared the wing, the farthest point is the rear of the tail, so you find the center there.