One way to keep the denominator constant is to duplicate the datasource and remove all the joins. Your secondary data source now becomes and independent data source. You can get the denominator from secondary source to calculate your percentages. Since the secondary source has no impact thru filters the value will remain constant.
Is the data really in Excel, or really in SQL ?
In SQL you can do what you want with a few subqueries (pretty easy to do).
If you're data is in Excel you can use Custom SQL, but it tends to slow/choke as the dataset grows.