How to Custom Aggregates for Group level of Crosstab

hello all,

i am having a small report (with crosstab in it)

Column Year
Rows Currency,Country,productgroups (here i am grouping by these columns for sub totals)
then factcells are a calculated weigted average —> total (sales * noOfItems) / total(noOfItems)

so till here i am good to go

so now the requirement is i need to calculate weighted average subtotals for each productgroups, country, currency

so i tried to selecting fact cells and then goto aggregate (on tool bar) then custom it says “Custom Aggregates only for lists”

so what can do here, how can i get weighted average for summary (group by levels)

please help me sir
Thanks in advance