I watched your video and tried to create the calculated column like you did in the video and it gave me a circular error. I Think there has to be a filter with the Date or some other reference Account ID to get the correct Months data associated with that cost.
Here is a sample dataset. Hopefully you can help me find a solution to get the Sales & Costs lined up so I can get the Gross Margin % at the detailed Account Level. In this example I want the C/S Co 26' Cost to be on the New-Co 263 line so I can get the % GM with that Associated sale. The 34.5% should be on the New-Co 263 Line. The Sales Line item data has that Account #2 which references the Cost line item I think this should be easy to reference?