Calculate table with loop

I suggest you change the structure of the original table in Power Query by pivoting the "Dimension" and "Quantity" columns as follows:

 I would then suggest you create new dimension tables for Item using:

Item Table = 
SUMMARIZE('Source Table', 'Source Table'[item], 'Source Table'[item description])

and for Year and Month following this pattern:

Fiscal Year Table = DISTINCT('Source Table'[FY])

Set up the model using single direction one-to-many relationships between the dimension tables:

Create the measures you need following this pattern:

Ordered - forecast month = SUM('Source Table'[Ordered])  - SUM('Source Table'[forecast current month])

Ordered - forecast m-1 = SUM('Source Table'[Ordered])  - SUM('Source Table'[forecast m-1])

Finally set up a matrix using the fields from the dimension tables and the measures. In the formatting pane, under Values -> Options, turn on the option "Switch values on rows" to get:

Attached is the sample PBIX file