Power Query. Group Rows Based on Periods and Insert a Calculated Row. Please See Exam

[Alienontherun]

Hi
Below is a short sample of a large dataset and what I am trying to achieve.
Thanks for any help on this.

 

[p45cal]

Could you attach an Excel file instead of a picture of one?
It looks like each row is a single cell but not sure, and we'd know without asking what's between the values in those cells.
Also, where are these data from? They look like they've come from outside Excel; Power Query could probably get them directly if they were say, on the web or in text/csv files, which would likely be more robust. If so attach those files as well/instead.