Really enjoyed this one and learned a great deal in the tool configuration department. Particularly enjoyed looking at some of the other responses and the different ways one can use to solve this.
Figured it out by stubbornly learning and sticking with the Multi-Row Formula tool trying to find a formula that worked ^^;;. I wish I could assign a value for the "Value for Rows that don't Exist" section but it defaulting to 0 was ok.