Actually the horn should be placed closer to being inline with the servo shaft, but not perfectly inline if you want the best mechanical advantage at full throws (both up and down). If anyone has a bigger picture than mine please post it. Mine is hard to see. Having the control rod at 90 degrees when the surface is at center gives you the most force only at center. At full deflection it is minimum and that's where you need it especially for 3D.

The other thing is that it's impossible to have no side load on the horn with that setup (or any setup for that matter). If that's a big concern go with the Dubro adjustable control horns that use threaded bolts for the horns. They can handle any side force you can generate. Actually those Chip Hyde ones look like they will be fine too. They are designed to handle some side force.