I don't think the PP understood your question.
If you have a party of 6 or more, an 18% gratuity will be added based on the cost of your food. The gratuity is not taxed.
I'm using even numbers for simplicity, assuming a party of 6:
If Food/Drink Cost is $100.00.
The tax will be $6.50 (6.5% based on $100 bill)
The gratuity will be $18.00 (18% based on $100 bill)
Total bill is $124.50.
A lot of people (including myself) will usually tip on the total bill that includes tax (which in this case would be $106.50 before gratuity), however WDW doesn't base the 18% gratuity on the total with tax included, only the actual food/drink totals.
Hope that answers your question.