Since you are only looking for an estimate of capacity remaining this should work:
DECLARE @DailyCapacity INT = 7950, -- Daily capacity
        @Minutes DECIMAL(18,2) = 660, --...

First, try reading up on things in books online, here is the entry for sp_executesql: https://docs.microsoft.com/en-us/sql/relational-databases/system-stored-procedures/sp-executesql-transact-sql.
Then, try this, no promises as I have nothing with which to test:

This doesn't really answer the question: https://docs.microsoft.com/en-us/sql/relational-databases/backup-restore/copy-only-backups-sql-server

All you can do is try it and let us know.