Constant Total column in cognos crosstab

while calculating percentage of contribution to the total by each individual department we need to keep total value constant.
For that i used following expression in data item .

total("Measure column" within set "Dimension column")-this gives constant total column.
example:
Total([bill hours] within set [departments])

Create one more data item for percentage values.
[bill hours]/total

                                                                                         The above images explains the scenario.