So I'm updating this post because I don’t think my initial corrective action was the best. Here's the Original problem/conclusion:

While sailing in about 15 kts of wind (with the Square Top Main), the knot in the halyard pulled through the V-Cleat at the top of the mast. It happened again a few days later.  I decided to mark the halyard at the cleat at the base of the mast at the raised position plus an inch to reduce reliance on the knot and v-cleat at the top of the mast. In this way the horn cleat near the base of the mast, not the V-Cleat near the head, was carrying the mainsail load.

I think it was not ideal to raise the sail an additional inch beyond the knot at the V-cleat. Why? The halyard doesn't stretch (by design).  My action stiffened up the mast quite a bit because the taught halyard runs along the front of the mast. In this condition when you tighten the downhaul to flatten the sail, you don’t get the designed mast bend.  I think the better solution is to make a bigger knot for the V-Cleat.  After you make the knot bigger make sure you still have enough room from the knot to the head of the main to be able to pull the knot down to clear the V-cleat when de-rigging.