The "drafting method" 8178 suggests works well and makes it easy to see what's going on. You can't go wrong with that.

Over the years there have been several formulas and even computer programs to do the same thing. For several years I've used Blaine Beron-Rawdon's "Linkage Design" program. It's an Excel spreadsheet that includes an interactive schematic display of the servo and control surface that you can move over the full range of travel and watch what the linkage is doing. It also computes the hinge loads and the basic servo requirements.

It's done well by me.

Tony