An OCMOD XML file to add fields to the admin so you can enter the price of a product including taxes, and have the price excluding taxes calculated for you using JavaScript.

Adds fields to the data, options, special and discount tabs of the product edit form, and shows prices including tax in the product list view.

My examples show 10% tax, but it uses whatever you've set up. You can base it on the store address or a completely different zone.