Honestly - I bend the joiners, mock everything up with the hinges temporarily installed and then trace were they need to be on the flaps or elevators with a felt-tip, then build my plywood "clips" and install them.
On a built-up flap I'll leave a 1/2 tip outboard, mock it up and then sand the ends of the flaps to even out any problems. With reasonable care you can get joiners pretty close ( say with .063") by just going slow and using your eyes.
Or lately, I buy horns and design the parts to fit. That's the easiest way to do it.
Chuck