Pivot Table Calculated Field

[Oprichnick]

Hello,
I want to create a pivotchart in which I want to show only the % of one item from its subtotal.
Using % parent I can easily calculate the %, however when I filter only that item it returns 100%. Of course, because now the subtotal is only summing its values.

In the below example: I want to have a chart/table in which I filter only "b" and what I have is the "b"%.

I thought about creating a calculated fields, but I am not being able to do it... What would be the best solution in this situation?

	1				2
	a	b	c	1 Subtotal	a	b	c	2 Subtotal
x
y
z

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Thanks in advance
Pedro

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