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Since 15-18% is considered standard, this advice would be generous, at least in my part of the US. kas23 is right that it would be acceptable in any restaurant. It most likely would be very appreciated, particularly if the service is special. (I'm not trying to start a flame war about proper tipping levels, honest.)

As a member of the food service industry (albeit one with a generous salary), I'd like to point out that for most servers, the tips are pretty much what they make. Most places pay $2 an hour plus tips, but my guess is that the $2 ends up getting withheld and the server/bartender ends up with whatever he/she makes in tips. Consider for a minute if your boss or your customers got to decide what percentage of your salary you make each day. That's basically what happens with a server.

That being said, I'm not averse to tipping based on service (i.e. 10% for lousy service up to 25% for great service, averaging out to 20% or so).
 
Tipulator is the best one out there.

I've searched high and low for the most aesthetically pleasing and functional tip calculator, and Tipulator is it. Also check out iHandy Tip Calculator; it comes in second place. Tipping Point is another good one (has the most options); I just don't particularly like the interface.
 
Definately go with App Box Pro. It has an excellent tip calculator and has many other useful tools all mixed into one dandy little app. Hope this helps you and have a nice time in the states.
 
How about your brain? This isn't a smart-ass answer, but just a little advice. Take 10% of the final bill and double it. It's very easy and will be acceptible at any US restaurant.

This. My mother works in Accounting and taught me this trick awhile back. Take the final amount, move the decimal point to the left 1 digit and then double that amount. Its crazy how easy it is, I can calculate the tip faster than I can pull my phone out of my pocket.
 
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