Just to make sure I understand correctly,
In your users table, you will include a discount column? So the discount percentage will be set on a user by user basis?
Are you using SecurityAssist for login?
If all of the above is true, on the login page, double click the SecurityAssist Authenticate user Server behavior.
On the third page of the wizard, you can select database columns to store in session variables when the user logs in. Click the Plus button and select the Discount column to store the discount percentage from the users table to a session variable
In the eCart discount rule for the calculation, replace 0.05 with $_SESSION['<Session Var Name>']
where <Session Var Name>is the name of the session variable.