Turn Over Tax (TOT) is NOT to be shown as a line item on a bill and has nothing to do with any service charge, gratuity, tip, whatever you want to call it on the Dutch side.

If any form of add-on is included, consider that a service charge and do what you are comfortable with when it comes to adding more. IF you ask, most servers will not be straight forward with their answer as to where it goes and you may be duped into adding way more than you would otherwise.

I agree with boucharda, add a little more IF you want and had good to excellent service but you are in no way obligated to do so.