report builder Percent of total column ?

Any one now how to create a percent of total column with areport builder report, or as a calculated value in the model?
I can manage it in a normal report ie regional sales value as a percentage of total sales by naming the total field on the report and referencing that field in a new formula ie regioinal value / TotalSalesValue
I can get an expression like (select sum sales value from mytable) into the dataview as a calculated field but this wont work to give me a total that matched the current report filter which it could do if i could get the correct expression into the model
This should be a common requirement but i cant find a decent answer online
Cheers
Colin