Pretty much the rule of thumb I use with respect to pole height is that the setup needs to be in the windshadow of the luff hull when that hull is just kissing the water. I "feel" that this is more efficient aerodynamically them having the snuffer setup stick out above the hull and thus increase surface area.

Apart from that a lower pole allows a longer luffed spi and that is always more efficient. But the pole should not be so low that its tip will hit the water when the bows dive down. I think on my boat this means that my spi pole tip is 450 mm above the horizontal line coming off the bows.

Wouter