Sum of Calculated fields in Pivot Table
 

I have a pivot table that summarises invoice values that are in different
currencies.
I added a calculated field to calculate the GBP value of each invoice, but
the pivot table seems to base the calculation on the sum values rather that
calculating first then summing the calculated field.

Eg
Customer Invoice value Exchane Rate
1 1500 1.5
1 2000 1.6

Instead of getting 2250 (1500/1.5) + (2000/1.6)
I get 1129 (3500/3.1).

Can I force it to calculate the invoice value first?