I originally solved it using Append Field and Distance tools, but the result was off for certain items. Since I couldn't find the reason, I took a peek at other solutions and found out how to use Find Nearest. Turns out my original solution was fine, but I used Required vs. Assigned for my total UnitMiles Formula, thus impacting my totals. I had assumed that the Required and Assigned were the same, which is something I should double-check next time.