once you combine the criteria get the logical sql query and frame it so that you get % calculation
that suppose to be some what outer query based on 2 inner queries and then set the sql and run the report.
I still did not get your clearly. Can you explain It more specific? where to set the sql?
Check this for query sample
where to get and apply; Answers->Advanced tab->look for SQL Issued
get the query using notepad edit as required and then past the updated query and then use the button to apply your changes and then Results tab