Subtraction between % of Grand Total columns in a Pivot Table as a calculated field

[pand23]

Hello, I have a pivot table which contains the following (summarised):

[TABLE="width: 398"]
<colgroup><col><col span="2"><col span="3"></colgroup><tbody>[TR]
[TD]Row Labels[/TD]
[TD]Rec'd @ Retail[/TD]
[TD]Sold @ Retail[/TD]
[TD]% of Rec'd[/TD]
[TD]% of Sales[/TD]
[TD]Desired calculated field[/TD]
[/TR]
[TR]
[TD]Nike[/TD]
[TD="align: right"]£300,000[/TD]
[TD="align: right"]£30,000[/TD]
[TD="align: right"]8.00%[/TD]
[TD="align: right"]2.25%[/TD]
[TD="align: right"]-5.76%[/TD]
[/TR]
[TR]
[TD]Brooks[/TD]
[TD="align: right"]£900,000[/TD]
[TD="align: right"]£500,000[/TD]
[TD="align: right"]11.18%[/TD]
[TD="align: right"]37.44%[/TD]
[TD="align: right"]26.26%[/TD]
[/TR]
[TR]
[TD]Grand Total[/TD]
[TD="align: right"]£3,749,208[/TD]
[TD="align: right"]£1,335,405[/TD]
[TD="align: right"]100.00%[/TD]
[TD="align: right"]100.00%[/TD]
[TD][/TD]
[/TR]
</tbody>[/TABLE]

The two % columns (4 & 5) are just the re-insertion of columns 2 & 3 with the "SHOW VALUES AS - % of GRAND TOTAL" displayed.

I want to be able to insert a calculated field that does the difference between Sales - Rec'd. I've manually worked it out above.

Is there anyway you can insert a calculated field that will subtract Sales & Rec'd as percentages? I've tried, but have no clue how to have dynamic calculated field that always looks at the Grand Totals at hand.

Cheers