Im not sure of the exact distance but most people say you should be able to grab the spin luff (leading edge) and turn it about 90 degrees in your hand. 180 degrees = too loose, 45 degrees = too tight.
Also, if the luff of the sail is always curled, it may be too tight, try moving your knots a bit too loosen it up.
In lighter air, I think most people ease off the halyard a bit, 10cm, to keep the spin full and flying more easily.
I love these calm moments before the storm, it reminds me of Beethoven...
'02 Hobie Tiger USA 1152