the billing state abbreviation is stored stored in a session variable named "eCartCheckoutForm_state_province"
you could create a recordset to look up the tax rate on the confirm page using the eCartCheckoutForm_state_province session variable as a filter, then add a set session value behavior (from the server behaviors panel click the plus button and select eCart -> General -> Set Session Value) to store the billing state from the recordset.
you will need to double check the code order to make sure the behavior to set the eCartCheckoutForm_state_province session valriable is before the recordset code, and the behavior to set the new taxRate session is after the recordset.
then you can use the tax rate session variable in the tax calculation.