I was reviewing the workbook and found a couple things that are throwing off the number off slightly. Using the Lookup function does work, but the view is displayed with only the bottom 20 showing, as stated above, the calculation is being performed on the values that are seen on the worksheet. They are not being calculated across the last 3 months. but the last 3 days. That is why we see the column of nulls stop after 3 (see image below, red arrow) and then starts returning values.
Do not fret! The desired calculation can be achieved with some help of some great resources and a LOD calculation. I would start with the discussions below and please reach out if there is a trip up trying to implement one of the solutions.
I am also going to try to put together a sample packaged workbook with a solution, but I wanted to provide this information first! Hope this helps!