Pivot Table Calculated Fields, Custom

Hi,

I have regular reports that I must send to a client. The report is based off of hundreds of thousands of lines of data.
We are able to produce a pivot on this data, and this is what we report on a weekly basis.
The historic data updates every week. (e.g. think in January there is a payment owed of 50k, and this shows up in January as 50k. It is paid in April, and then the data shows the payment owed in January as nil)
This means that the data for the report cannot just be added to.

My task is based on the below.
At certain stages we are able to consolidate the data into subtotals of columns. This happens at random dates based on certain criteria and remains consolidated at these points.

In the sample table, I have set this out as showing the data supplied on a monthly basis.
Like the data and reporting, it is not reported on actual dates where you could organise as months/quarters/years etc.
For this sample, what I am looking to do is to create a calculated field after the third month (2021-03)
I would be looking to subtotal each of the Value columns at this stage, and not at the end (0221-05)
So for instance, Field 1 should calculate Sum of Profit for 2021-01 + 2021-02 + 2021-03.

I have provided in the attachment data, The Pivot, and a sample of what I am looking to achieve. (see M32:O51)