DAX Percentage to find proportion against a slicer with a date flag set

[DebbieE]

Im really struggling with a bit of DAX at the moment

%  = [Number ]/CALCULATE([Number ],ALLSELECTED('Metrics'))

this is great, obviously until I filter

 

I want to see them all. As an Addition I have a Filter on this visual containing a flag for Previous year against the date table

I tried this DAX 

% by month (against All ) = [Number]/

CALCULATE([Number],

ALL(DimTable))

And this worked  fine without the flag 

But when I add in the flag from the date time table it goes wrong

this is obviously because I dont have anything about month in the DAX.

I have tried a number of things. ALLEXCEPT etc and I just cant figure out how to get this one working

Any help on a solution would be appreciated, I feel like Im almost there