I'm inclined to say the latter of $z from $x since you calculate the discount in dollar terms and then adjust for it (generally speaking), BUT the 'tax adjustment note' is throwing me off as I'm not sure how it works in this case. I suppose the only difference between your two proposed options of y% or $z is based on timing and really will only form a complication when you have multiple discounts or something being applied (e.g. a 25% off sale on goods already 50% off does not mean you get 75% off).If the original price is $x, the discount on the tax adjustment note is y% and that equals $z. Would you subtract the y% or $z from $x?
Thank you