Business logic isn't pulling data from database, how to map an object to a table, or the specific SQL query to grab some data. Those are technical details. SQL should just be pulling, saving and querying from the store without much maniplation. Not changing the data.

It's the rules that a business stakeholder might be interested in creating.

When you start putting the tax calculation rules into the stored proc, thats where the problems begin.

If you create a CTE to pull the information required to perform the calulation then return that info without doing the calculation thats fine.