Crockett:

There are three ways to address this. First is the simplest. If it's not too far off, forget it.

Second is still simple. Sight down the hinge line, consider the control surface being at 0 degrees, look at the angle a line would make from the hinge line through the connector hole in the horn. If that line, for example, leans back at 80 degrees instead of the 90 you want, then set your servo arm at the same 80 degrees. If the servo arm and control horn are the same length this will bring the operating angles right in. Uneven lengths? It'll still be pretty close.

The "Right" way is to get control horns with a greater offset. But they cost more.

Unless it is a pattern ship, or you want the utmost accuracy, go with option one or two.

HTH.

Bill.