Take your measurements from the TE of the fin/rudder right from the plans. Measure forward to the hinge line. Cut and cap. I do this step with the LE and TE's and the tip installed and sanded.

After you mark the hinge line, measure from the bottom at the hinge line, and the same distance at some point forward for cutting the fin to size.

It is noteworthy to not cut away the capped portion of the fin at the hinge line, but instead add a piece of 1/4" to build up the chord slightly.

I hope that makes sense. If not, I'm about to do one myself so I'll take more detailed pics of the process.

-Mike