I pretty much fit the spi gate in the mast which is limited by the F16 rules to 7,50 mtr above the top of the main beam and by the advice giving by Goodall yachts (is the same)

Than on a day with a mild wind. I fit the pole with its end about 0,5 mtr. beve the bows and hoist the spi and see how it sits. Adjust your pole to the cut and luff curve of your spi. Often this means moving the end of your pole up or down a few centimeters (max 15 cm or so)

I'm unfamiliar with the Goodall 17,5 sq mtr. spi so I can't help you there. Of course Goodall Yacht sails themselfs might give you a good starting point / height

Wouter