SPLAIS-I usually draw it out on a piece of paper with length, horn width, distance, etc. all matching my set up.
Then I tape it to the bottom of the fuse and mark the exit points.
Next I use tape and make a line from the servo arm to the rudder horn on the out side of the plane.
Where the 2 points intersect you have the exit. Then widen the exit along the tape to your preference.