Please disregard the above post (timeout while editing - lost connection).
The "Onetime Discount" module was intended to be used when a store owner needs to offset the entire cost of an order due to a problem caused by humans at the store. For example failing to meet the expected shipping date. As such it does not alter the product pricing or taxes. It simply provides a simple way of handling issues caused by human error (the alternative workflow is creating "store credit" and applying it against the order).
The order total module ot_tax applies taxes based on product price, so you are correct. The tax will always reflect the tax rate against the product price listed in the order.
What is the situation requiring the order be discounted? Does the situation occur often? Can the store owner just move the sort_order to be after tax and add the amount of the tax to the actual discount they manually add?
Hopefully the above will work for the store's situation (I am curious about the situation, so examples are welcome). I'd also be receptive to contributed code adding "tax recalculation" to ot_onetime_discount (preferable checking if ot_tax is enabled, what the sort_orders of the two modules are, and if any other modules have already altered the pricing / tax).
