@Andy May ，
I am sorry for the thoughtless reject of the question. My thought was that your question description is hard to diagnose which part went wrong, since the DAX query / data model / details are not quite clear, and this forum is not professional with the PBIRS/PBI design. Also I have not understood the "percentage column" in the description.
From the current description , we want to find the subtotal percentage of a measure, right ?
You could try directly calculate the subtotal percentage in the tabular model instead of adding them in SSRS/PBIRS.
You can find similar request in these blog Calculating Percent of Subtotal or the DAX expression in this thread :Percentage of subtotal
If you still find no solution for this case, you could provide some sample data and your query expression so I could do some test and offer more specific help.
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