All sails should be as low as possible so you do not give it all up to righting moment. That said there is also the aspect ratio of a sail which is actually a wing. The higher aspect seems to be more efficient.

I think that you would like the spinnaker as low a possible but you do not want the pole hitting the water or the waves at all in any conditions. The height of the boom seems like a nice place to start as far as how high the tip (tack of the spinnaker) should be.

Dan