The tips on the bill that they 'suggest' are based on the pre tax total. They were giving 3 different amounts on a lot of the bills this time, 15%, 18% and 20% if I remember rightly.
We usually get a Tables in Wonderland card and the automatic gratuity built into it is always pre-tax so when we go outside the Disney bubble and have to pay the tip OOP we always do it pre-tax.