Dax Formula with filter All Selected

Hi,

 I am struck with this Dax calculation I could use some help filtering out this cumulative sales calculation to get the monthly averages. I was thinking adding in an "Allselected" filter into this Dax formula, but I can't figure it out.

Here is the data when I make slicer selections, and the data seems to be working fine. But when I dont have any selections, The data comes in negative and is wrong.

I was able to get the Averages working on the Year page, but now I want the Current Month. Which I need help with this Dax -- I was thinking an "All Selected" Filter or something???

Thanks for any Help!

Sales $ CM =

VAR __PREV_MONTH =

CALCULATE(

        [Sales $ CY],

        DATEADD('Date'[Date],-1, MONTH)

    )

RETURN

    

    If(

If( [Sales $ CY] <> Blank () ,

([Sales $ CY] - __PREV_MONTH),

Blank()) = 0,

Blank(),

 

If( [Sales $ CY] <> Blank () ,

([Sales $ CY] - __PREV_MONTH),

Blank () )

)

 

 

Here is the Average When Selected:

Average When Selected

 

Average When not selected:

Average Unselected