Quote Originally Posted by lat9 View Post
... there are no tax-related calculations performed by the 1-time discount.
Correct. Taxes are handled by the order total for tax (ot_tax).

If you place the ot_onetime_discount sort_order before ot_tax, then the discount is pre-tax.
If you place the ot_onetime_discount sort_order after ot_tax, then the discount is post-tax.