Calculated field - count values in columns and divide by grand total

[igneous]

I'm working NPS (Net Promoter Score) reporting for my company. The Pivot table looks like this:

Sum of WEIGHT	Column Labels
Row Labels	1	2	3	4	5	6	7	8	9	10	Grand Total
oktober 14	1		0	1	10	1	7	5	2	11	38
november 14	0			1	10	4	3	4	1	9	33
desember 14				1	1		1	4	0	7	14
januar 15		1			4	2	4	15	3	8	38
februar 15	1	0	0	0	7	1	2	14	4	7	39
mars 15	1		0		10	3	3	7	1	7	33
april 15	0	0	1	1	5	1	2	5	2	9	26
mai 15	0			0	15	3	4	4	2	6	34
Grand Total	4	2	2	4	61	16	25	59	15	66	255

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I need to calculate four fields within the pivot table itself.

1. Detractors: sum of 1-6 scores from column labels, divided by row grand total
2. Neutrals: sum of 7-8 scores from column lables, divided by row grand total
3. Promoters: sum of 9-10 scores from column lables, divided by row grand total
4. NPS: Promoters - Detractors.

I can obviously do this outside the pivot table, but the calculations get messed up if the pivot table changes. So I am looking for a way to do this with calculated fields. An example Excel file can be found here: https://www.dropbox.com/s/xew1i5ltrh3ui8j/NPS.xlsx?dl=0

Any advice?

 

[igneous]

Anyone?