The simplest way to get symmetric throws is with the horn pickoffs exactly on the hinge line as you guys say, but it's NOT the only way. If you match the spacing on the control horn and servo arm and offset the pickoff points by the same angle as the offset in the control horn vs hinge line, everything matches perfectly. It's a pain in the tail to do............. but it works. I did it on my Excelleron rudder, just to keep from buying an extra $25 in parts and throwing all but the extended horns away.
Gordie