Hi Chris,
Let me see if I can shed some light on this. I bought my spinnaker rig from Jeremy at Surf City and am fortunate enough to live with-in driving distance of his shop and he was kind enough to help me rig it the first time (actually, the cat in his spinnaker rigging video is mine). The problem was that between that first time he helped me and the next time I rigged it on my own, I had forgotten the proper routing of most of the lines. This diagram in the below link helped me tremendously and even though it shows a Hobie 16, the spin on an 18 is virtually identical.
http://www.hobiecat.com/support/pdfs/20999020.pdfThe only differences are as follows;
1) Looking at the diagram, item #3 shows a block with spring and eyestrap sewn to the trampoline. There is no need to sew anything to the trampoline. We simply tied a short length of 1/4" bungee cord to a single block and then passed the other end of the bungee through one of the grommet holes at the rear of the tramp and tied a stopper knot to prevent it from pulling back through. Your retrival line comes out of the aft end of the spinnaker bag, comes up through the 1st or 2nd center tramp grommet just aft of the forward crossbar (as shown in the diagram), then passes through the single block you have tied off near the center aft end of the tramp before going back forward through the cleat mounted on your forward crossbar. When completed, your halyard/retrieval line will look like a "V" sitting on top of your tramp.
2) The spinnaker sheet blocks are mounted to the bottom of your shroud chain plates, not forward of the chain plates as shown on the diagram. I suppose the location of these blocks depends a lot on the cut of your spinnaker but I think they'll work fine here (bottom of chain plates) with the Tiger spinnaker.
The retrieval line DOES pass through the grommets in the center of the sail. In my case as well as most I'm sure, there are 3 grommets. Pass the line through the bottom grommet, back through the 2nd grommet and finally, through the 3rd (top) grommet. Tie a stopper know in the line after passing it through the top grommet only. There is no need to tie stopper knots anywhere else but the top grommet.
You were right in saying that there needs to be just enough slack in the retrieval line to allow the sail to completely fill. I rigged mine several times in the driveway ensuring my lines were the perfect length before taking it out for a sail. It all seems very complicated until you do it a few times.
Hopefully this helps...don't hesitate to ask questions if you need more clarification.
Tom