I struggle in very light winds also! I have seen somebody run their halyard out about 6ft and run really deep... the sail is very unstable though.
The Goodall kites I have seen are all very tight along the luff... no slack!
My Goodall spinnaker has a luff tension line.
Robbie Daniel told us that luff tension should be 1% longer than the distance from the pole to the top of the hoist hieght. I think most people are using a stopper ball to make the height at the top adjustable, and also raising/lower the pole to get the number right. The folks at CRAW, (Catamaran, Racing, Association of Wisconsin), swear by two fingers grabing onto the luff and turning it 90 degrees.
