By Rod Beurteaux
The issue with asymmetrical sails is that the tension in luff of the sail determines the optimum angle the sail at which the sail should be used. In very light airs, the Viper will soak downwind if you can keep the spinnaker set and drawing. To achieve this, the sail needs to rotate to windward and the length of the luff is the key to how far sail will rotate.
The window for when to do this and by how much depends purely on the wind strength. In too little wind, you need to sail higher just keep the sails set. In too much wind, the boat will sail too slow when soaking and reaching higher with a tighter luff produces a better velocity made good (VMG). When the conditions support soaking the boat as low as possible the sail luff should be lengthened to allow the rotation. In that adjusting the tack line to lengthen the luff is impossible with the upward loads between the pole and the hull, lowering the halyard a foot or so is the solution. To get a handle on what works best is to try it.
A couple of extra points: we do not worry about pulling the halyard up through the gybes. Make sure the sail is well rotated the clew is well out from the boat and in a good position for the windward sheet to take the new load as the boat is turns through the gybe. (Note: this is for a float gybe and not a blow-through gybe.)