You have twin rudders, with a connector rod synching the two tillers.... the tiller extension connection is your "extra pivot point".. is that correct? So the 18" you're concerned about does not have to be exact. What you really need to be focused on is the amount of swing distance the pin placement will allow. You can mount the pin to a single tiller, which will turn both... or you can mount the pin to the connector.... it doesn't have to me dead center... it can be anywhere along the connector... .so long as there's enough clearance for the TP to extend and retract the distance needed to function properly. There's a good possibility the tiller mounting to one side or the other may not allow full extension and retraction, because of the TIller Pilot's case length taking up too much space. Or maybe the connection rod is too far forward... well past the recommended 18". In this case you may want to mount a second connecter at a more desirable location on the tiller.
The point is that you may have to experiment a bit... plug in your TP , use wire or tape to connect the arm to various locations., and start playing with it. The problem will soon solve it self. Don't start drilling holes till you've tested a number of locations. Take some pictures of your success and post them for the rest of us to admire. Good Luck.