Tax Locations - Add Field?

Hi,



I need to be able to add another criteria field to the Shipping Locations other than:



Countries

States

Postal Codes

Cities

Addresses



Is there a way to do this easily?



The fields exist in the software already, I have added them to user profile fields for shipping on checkout.



Please let me know,



Cheers

So I have added the field, and the field inserts/works correctly.



Where does the calculation for location and tax rates take place?



I now need to add an if statement based upon new field.



Cheers