How can i refer Max sales Date within Server Expression ?

Hi All,

I wanted to adjust one of our custom series:

Example: We are in March Cycle currently, and this series X has to refer Actual/History based upon Max Sales Date, if it FEB'16 or Before it has refer Actual,  if it Post FEB'16 it has to refer Forecast (this will be based on Max Sales Date which will refer from Sys Param). Can anyone tell me how can i achieve this ? I was trying to give Sys Param Table in Extract From while creating Server Expression, but it is not allowing.

Thanks,

Sharath Chander M