Text field equation turned off by checking a checkbox.

I have an invoice with a section that includes a "Tax" field. I have a simple equation in this field that calculates the tax based off the amount listed in the "Total" field. The equation simply reads: "Total * 0.0825"

However, I want to be able to select a checkbox called "TaxExempt" that will make the value in the "Tax" field be "0.00".

I've read every forum, but can't find specific solution for this problem. Help!