AJ, please have a look at pg 198 of the CFA 2013 equity book -the first paragraph and under note 5. The reason interest in after tax is because the WACC has a debt component which is after tax. You could add it back before tax, just like your depreciation but then your WACC must not include any tax shields on debt. Either way, you’ll get the same answer! The key is consitency.
To all those whom don’t trully understand, and I’m not trying to be concieted or above you, I hope this helps you too.