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The cloth device you describe (probably) allows the tack of the Gennaker to slide up and down on the furled headsail. Alternative devices to do the same thing: Parell beads, ATN "tacker", etc.
If your questions is: How high above the deck do you set the tack of the Gennaker (asymmetrical)?
Answer is: On a set/specific course, slowly ease the spinnaker sheet until the luff edge of the spinnaker begins to 'curl' ... and watch exactly where that 'curl' in the luff starts .....................
The goal (for that angle/course) is to have the 'middle' of the spinnaker luff 'break/curl' in the middle or slightly higher than the MIDDLE of the luff forward edge. If the luff 'breaks/curls' high up near the head of the spinnaker; then, the sail is being flown 'too high' and you should tension the tack line and bring the sail down closer to the deck. Conversely if the luff breaks/curls below the midsection, then the tack line is holding the sail too low and it should be eased .... until the sail breaks/luffs/curls in the middle or slightly above the middle.
Second consideration (dependent on wind strength and sea-state .... the higher the tack line allows the spinnaker to 'rise' the more unstable the sail will become.... unstable to wave action versus the boat and unstable to just a few more degrees of wind shift. Generally the lower sailing angle downwind (down to about 135deg) you sail the higher you carry the tack; conversely, the higher the sailing angle (up to a close reach if the sail is so cut) the lower the tack is held. ..... but the 'key' for good basic sail efficiency and stability is getting the sail to 'break/luff/curl' in the middle of the luff ---- and the that is controlled by the tack line attached to the tack of the sail.
The above is whats known as an 'elementary' setting/shaping and once you master that and how to control ......... the ultimate step is to get most of the 'lift' out of the spinnaker so that the sail begins to LIFT the boat's bow out of the water and not just 'push' the boat downwind, etc. ........ . :-)