How can I get a percentage field with power pivot?

This should be a fairly easy question for Power Pivot users since I'm a newbie. I am trying to do the following. After pivoting a table I get a crosstab table like this

rating         count of id
A              1
B              2
Grand Total    3

You can imagine the original table only has two columns (rating and id) and three rows (1 id for A and two different id's for the B rating). What DAX formula do I have to write in order to create a measure that simply shows

rating         percent of id
A              1/3
B              2/3
Grand Total    3/3

By 1/3 of course I mean 0.3333, I wrote it like that so that it is clear that I simply want that percent of id is the count for each rating divided by the total count. Thank you very much

Solution

You need to divide the count for each row by the total count.

DIVIDE (
    COUNT ( Table1[ID] ),
    CALCULATE ( COUNT ( Table1[ID] ), ALL ( Table1 ) )
)

For this particular calculation, you don't have to write DAX though. You can just set it in the Value Field Settings.

Summarize Value By  :  Count
Show Values As      :  % of Column Total