I am looking at your module and it does seem great. But I have a query (in case I have set the thing incorrectly).
In the UK, Vat liable goods carry 17.5% tax. I have these products showing tax inclusive price on the product page. If I want to give a discount on final price of say 49 cents, I have to set the order total to 42 cents and then the checkout discount shows as 42 cents - which is correct before tax is applied to the lesser price, giving a further price drop. That is, giving a 42 cent discount also reduces tax by 7 cents giving an effective discount of 49 cents. (It works and displays fine with tax free goods). I have the module settings Include Tax "false" and Recalculate Tax "Standard". ( The other settings give incorrect results.)
Is there any way of showing the discount including the tax drop element? e.g. "discount including tax where applicable"