As has been suggested elsewhere Pete
The strip of (spinnaker weight is good and there is Arun Sails in Bosham - easy) cloth is best tied to the highest point where it can get to sniff the breeze uninterrupted - this is usually the 'peak', that is the top of the yard:

I originally put this one on 'Rosie Mae' just for fun, but it actually gives a very accurate idea of when the top of the sail is drawing properly as it runs round and hides behind the sail as soon as it is sheeted in too far.
CW