Good question, but hard to answer without going up the mast of a few boats. "I" is defined as the height of the upper bearing surface of the headsail halyard sheave. I suspect the forestay attachment point is higher, though without measuring, I can't say it is two inches. For comparison, the spinnaker halyard is 35 feet above the sheer in the Class mast spec. That's ten inches above the forestay attachment. Seems excessive, but my eyes are looking at this from a bad angle, 30 feet away, standing on the foredeck.
