MDX query help - how to get first 6 months amount for each person?

I'm trying to get some stats about new business.

I've used Chris Webbs existing method to get a count of new and existing.

However I now what to get each new customers business for 6 months and then display it by year.

So if the new customer starts at the end of the year, some of their business is allocated to the total of that year, and some the following.

I have the two sets of customers:

WITH SET ExistingInv AS
NONEMPTY(
NONEMPTY([Investor].[ID Key].Children
, [Measures].[Amount])
, {[Measures].[Amount]}
* {NULL : [Tran Date].[Fiscal].CURRENTMEMBER.PREVMEMBER}
)


SET NewInv AS
Except(NONEMPTY([Investor].[ID Key].Children
, [Measures].[Amount]), ExistingInv)

How can I then work out each investors first trandate and then get a six month total and allocate it by year?

Is this possible?

Any help would be appreciated.

Thanks

Jon