Cognos cross tab percentages

How to get percentages in cognos cross tab

I need to calculate % for count distinct value. to do this in the query we need 3 data items.
data item 1 – count (distinct item_id) . solve order 1
data item 2 – count(distinct item_id for report) . solve order 2
data item 3 – data item 1/ data item 2

use data item 3 in your crosstab and format it as percentage.