Made this tonight, but with an attempt to scale it down - I prefer my cocktails much closer to 3 oz than 4. I see what Mr. Kilgore was trying to do here - there's a commonality to rye, Benedictine and Green Chartreuse in that they're all green aromas and flavors. So mine was more like 2 oz/.5/.5, and it needed..... something. I tossed to mint in to the stirring pitcher, and that helped somewhat. Maybe some Fernet would be helpful to dry it out, as in the Monk's Revenge?
Like the 1st commentor, I, too, prefer a 3 oz cocktail rather than a 4 oz one. However, I made "Purgatory" using the given recipe because my mind said "yes, this will be a great drink." And it is, although the combination of Benedictine and Green Chartruse overshadow the Rittenhouse Rye--which should be no surprise. If I were to reduce the size of this drink, I would start by reducing each ingredient by 1/4 oz, and adjusting according to taste. But do keep the lemon twist! The challenge when reducing the size of this drink is to keep a balance of the ingredients' taste, but also, if possible, to bring out the rye, so that it is more prominent.