Hound height = distance base of mast to place where stays (shrouds) are attached to the mast.

I'll give an example :

mastheight = 8.5 mtr
hound height = 6 mtr.

spi gate = (8.5-6.0) *0.6 + 6.0 = 7.5 mtr.

This formula accurately describes the bail heights on F18's ,F20's , F16's and as good as all cat that have been retrofitted with a spi like Nacra 5.5; Hobie 18's, Taipan 4.9's etc. It will hold. In your case as nacra makes rather flexible mast you must remember not to release the mainsheet when blasting along under spi. USe the traveller to depower instead and otherwise choose to go over and go for a swim. Keep your mainsheet sufficiently tight, this is easy to spot, when you top bends forward more than just a little your mainsheet is to loose.

Good luck and have fun.

Wouter