This is one of my favorites, and the shortbread garnish is genius! But tonight I didn't have any shortbread, so I decided to experiment with an orange peel twist, and compare recipes where Cointreau or Drambuie were substituted for the Benedictine. Winner: Drambuie. Cointreau was a little too sweet.
Adapting from some other recipes I found online, I substituted 1/2 oz dry vermouth, 1/2 oz Cocchi Storico, 2 dashes Regan's orange bitters, and increased the (real) grenadine to 1/6 oz, and it was delicious!
Chris, I think Manhattans are better balanced with a 2:1 ratio of whiskey to vermouth, rather than the 3:1 suggested here. I also like them with a bit more syrup from the cherry jar -- maybe a barspoon. I don't know what you were using for ingredients, but better ones will of course make a better drink. No matter what brand of vermouth you use, it needs to be fresh. Vermouth goes stale after a few months once it's opened, even if you keep it in the refrigerator.