Changing the option to use the Store Tax Address is an option in the OpenCart 1.5 Administrator that is used when no users are logged in to the storefront.  It can also be set to calculate tax based on a customer's shipping or payment address.  This tutorial shows you how the option is changed in the settings for the Administrator Dashboard below.                                       

Changing the Option for Using the Store Tax Address in OpenCart Administrator Dashboard                                                                                                                                                                                                                                                                                                                                                                 

1. Login to the Administration Dashboard as an Administrator
2. Hover over the menu bar where it says SYSTEM and then click on SETTINGS in the drop-down menu
3. Select the STORE that you wish to edit and then click on EDIT on the far right to proceed
4. Click on the TAB that says OPTION, then scroll down until you see the option labeled USE STORE TAX ADDRESS as shown below:
   
   
   
   
   
   
   
   
5. Using the drop-down menu, change the option to NONE to use the store address to calculate taxes if no one is logged in to your Opencart storefront.  Or you can choose to use the Customers shipping or payment address to calculate tax based on their location.
6. Select SAVE in the upper right hand corner to save your change.

Using this option allows you to determine how tax is calculated if no one is logged into the storefront.  If you need to determine how tax is calculated with a customer logged in to the storefront, please refer to Changing the Option for Use Customer Tax Address.