Not to be snarky, but whatever you feel the service was worth. If you feel you got more/better service than you expected to receive for the standard gratuity, it's really up to you what it's worth.
There are some people who feel an extra $5 is plenty, while others feel $50 or $100 is more in line with what the service was worth to them.
A good/fair tip is the standard gratuity, that is why it is the standard. I would just use that as a baseline and on your last full day you can decide if you felt the service was as expected, a bit more than expected, or far above and beyond. Don't forget that the comment card at the end of your trip is just as important as any additional cash you give them. A good tip is great but positive comments can lead to promotions and pay raises.
In all fairness, I think many people go with an expectation of excellent service and judge from there. The point about the 'suggested gratuity' being for good service, should be remembered. Good service anywhere, versus good service on Disney, which would be excellent service anywhere else.
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