Jeff
You only need about 3 inches of slack. A little less than half a hand turn.
If its a separate tack line setup let the tack loose and measure by pulling the tack of the spinnaker tight below the pole.
We are using 2 inches on the F18, but these are a flatter cut kite.
In stronger winds you can drop the traveller down to the footstrap, but keep the main on and you will maintain good height and speed without having to blow the kite, if you need to gain height.
But they are a full cut kite, so you will have limitations on tight reaching. Letting the tack go 6-8 inches will assist with tight reaches also, but I prefer dropping traveller.

Goose