This might be a little tricky to deal with like this. The problem is that the tax is calculated based on the taxable total. The taxable total is the subtotal from the cart minus the discounts plus the charges.
The grand total makes use of the taxed subtotal. So if your discounts are not taxed then they will not be included in the grand total.
To make it work like this I think you will need to update these functions in the eCart definition file in the WA_eCart folder. In the functions for GetTaxableTotal and GrandTotal you would need to make adjustments to account for the setup you have described.