This is something that I really never understood.
Here im Portugal, and any of the European coutries that I often visit, either in work or leisure, I've never had the need to "calculate" a tip. I noticed that In Mexico and some South American countries, whoever is behind a balcony, table service, etc. is allways expecting for a tip too. It's institutionalized!!
I only tip the clerk if I really feel that the service provided was, at least, the one that I expected to get. And only for the amount that i decide to give.
Everybody considers "tipping" somekind of bonus, or prize, for a great service provided. Not something that is just guaranteed.
So, this kind of APP is completly of no use for me!
Unless i get the chance to visit the US, something that i've never had the chace to do.