Calculated field in Pivot Table giving weighted average

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When using a data table it is fairly straightforward to get a weighted average. In the example it would be SUMPRODUCT(B4:B9,C4:C9)/SUM(B4:B9).
The problem I have is the data shown is a pivot table and the column 'Proportion' is actually a value that the PT is showing as a percentage of the total. I cannot find a way of referencing the percentages in the column 'Proportion' in a Pivot Table calculated field, any formula gives the underlying value.