Dynamic Rolling 12 Month Based on Prompt Starting Period

[Tore2015]

I am in need of help with resolving this rolling 12 month issue.  First of all I am using a DMR as the OLAP source.  2nd, the report will need to default its 12 month rolling period to the current period, and then give the user the ability to change the starting period for which it will then show 12 months back from that point.

So far I am able to create the rolling 12 months using the following expression: 

tail(filter(members([Presentation].[Period Dim].[Period Hierarchy].[Month Year]),
tuple([FTE], currentMember([Presentation].[Period Dim].[Period Hierarchy]))
is not null),12)

This allows me to set the statis 12 months based on the first member of the time dimension that has FTE values.  But now I need to imbed the prompt macro to replace the currentMember..and I am having difficulty doing this  :( :( :(

If anyone can spare a moment to assist, that would be GREAT!!  Maybe there is a better way of accomplishing this....

Thanks