calculation with first day of the year.

I'm having a measures defined on a yearly level (connected to the 1st january of each year).

I'm having a calculation which defines a result (ratio) for each day (lowest grain)

Now I want to multiply this ratio with that yearly measure but make it applicable for my entire period dimension.

When I "hardcode" the formula like "[Period].[date].&[20130101],[Measures].[ForeCast]" it works.

But ofcourse I want to make this "[Period].[date].&[20130101]" dynamic based on the selected member of the dimension period.

Is there a way to do this?

Thanks