I guess I should assume it didn't work. Again, I'm not sure how SQL expressions work in 9.0, but if you have the option of using one, try creating the following {%sumall}:
(select sum(table.`duration`) from table)
If you have selection criteria, they would need to be built into the SQL expression.
Then create a formula {@percent}:
{table.duration} % {%sumall}
Add this as a summary field, using sum as the summary.
If the SQL expression doesn't work correctly, you could also return the sum of duration by using "Add Command" as your datasource.
-LB