On a calm day put your spi out and sheet it in all the way. Look at the sheet and the line it forms from the pulley to the clew. Basic measurement is to make sure this line crosses the luff at 90 degrees or so. It is usually better to have the pulley too far back than too far forward as you really don't want to close the gap between the spi and the main by pulling the clew down, so if in doubt, move it back.

When I changed spi from the original Stealth 'R' cut to the GP F16 one I had to change all the geometry of the front triangle: despite being the same size (17.5m2) the new spi is way flatter, with a longer luff and a clew set lower. I had to lengthen the pole by 40 cms and move the sheet pulley forward quite a bit to be able to get a good shape in it.