Actually you want the luff to be softer in order to sail higher (i.e. reaching)...it's contrary to what you are used to with hard luff sails and I was corrected on it some time ago myself - but consider this; as you loosen the luff of the kite, the whole thing will rotate to leeward moving farther away from the wind - this means that you have to sail higher in order to keep air in the spinnaker. If you tighten the luff, the sail is rotated more to windward meaning you can sail deeper and still keep it breathing properly.

I know the I20 guys position their halyard stopper balls to set the luff tension on their kites so they can grab the luff with their fist and rotate 90 degrees. We've been doing the same thing on my Nacra F18 and it seems to work pretty well. In order for me to get that luff tension on my Nacra however, I have to tie the tack and head knots as small and tight as possible to get a full hoist because both corners bottom out just when the tension gets right.