For myself, I find 1/2 oz of Benedictine overwhelms the Maker's Mark bourbon. My suggestion: (1) Make the drink according to the recipe, but add only 1/4 oz of Benedictine for start; add more to suit your taste OR (2) Make the drink according to the recipe, but add 2 1/4 oz of Maker's Mark bourbon, and add more to suit your taste.
I adjusted the recipe by adding a little more than a 1/4 oz of Maker's Mark. Unadjusted, I would rate it a four. After adjusting the drink by adding 1/4 /oz bourbon, I rated the Manhattan with Benedictine at a 4.5. This drink might also benefit from an orange twist.