Here is something that you might want to consider:
Looks like it's rated for 275 sq ft at mid-boom so it should work for your application. What's neat about it is that the fine-tune is fixed to the ends so when you adjust with it you don't run out of purchase. The best of both worlds for sure! Sailnet has a very attractive price on it (click the add to cart to see their retail price).
Was doing some checking on my boat's mainsheet; it appears (via Harken's calculator) that the double block that attaches to my traveler is not big enough either
Rated at 2500 Lbs SWL; I'm sure we are exceeding that by a bit when we get out in the heavy wind. While not severely undersized; it will be upgraded sooner than later. My boat has 3/4 boom sheeting with 300 sq-ft main.
It's possible that the Harken page is calculating mainsheet load in the event of a round-down (IE if you are 90deg to the wind and generating lift). It would make sense that they give maximum possible load numbers to be sure the product you pick cannot fail under most circumstances.