Dividing data in a table x(filter=p)/x

Ok, 

I have table ATstreams where i want to divide a metric by the same metric except segmented on another column.

Simply in psuedocode its Streams where [pname] contains "Extras" / Streams

So step one i create a new column called [extra streams], thats fine

Now for the life of me I cant create a measure or column or whatever that does [extra streams]/[streams], i get all sorts of figures.

All of this is presented rolled up in a table of course.

I even tried this in one step to no avail:

%Extras =
CALCULATE(sum('ATStreams'[Streams]),
RIGHT('ATStreams'[PName], 6) = "Extras") / 'ATStreams'[Streams]