Any time there is a discount on a meal, the tip should always be at the pre-discount price. Disney has done this for many years on the CM meal discount and started this yeqr on DDE.
However, the sales tax is computed on the amount after the discount is computed.
Example
!00.00 Meal
-20.00 Discount
$80.00 Subtotal
+18.00 Gratuity based on $100
+5.20 Tax at 6.5% based on $80
$103.20 Final Total