ExcelBanter - I want to create a calculated item based on a calculated field

Dear

I want to create a pivot table as below to compare sales numbers by product
with 2 years in the columns, and for each year the kg sold, the margin and
the margin/kg. Furthermore I want to calculate the difference between the 2
years for these 3 fields (kg, margin & margin/kg). I have created a
calculated field for the margin/kg.
For the difference between the 2 years, I have created a calculated item.
This is OK for the kg sold and the margin, but for the margin/kg, it gives
the wrong output (it calculates the margin/kg of the difference instead of
the the difference of the margins/kg).
In my example below, the pivot table gives for most right column 3,33, while
it should be -5 (= 15-20)

2008 2007
2008-2007
kg margin margin/kg kg margin margin/kg kg
margin margin/kg
product A 10 150 15 7 140 20
3 10 3,33

Thanks in advance
Stijn