I do 10% with decent service. People here don't tip very much. When someone says AR is not a tipping country, I think it's more like tipping is not completely uncommon (though Violet would know more than I) but it is small for the most part when they do tip. When I go out to eat with an AR friend, he or she also asks how much we should leave for a tip and at first were aghast at 10%. But it seems the more money and Argentino has (in general, not at the time), the more tip they leave as well.
Like most others posting here, I can't not tip, or just tip a tiny amount (except the one time I tipped a single peso because I tried for half an hour to get drinks at our table halfway through the meal and the waiter never brought them. When I paid and left 1 peso, the guy actually followed us out of the restaurant asking me if I realized I had only left a peso and I told him exactly why. It was a touristy/after-theater place near Corrientes and Montevideo named Chiquilin).
I can't help but think that 20% is a bit much (even in the States I felt that; my usual limit in the States is 15% unless the service is exceptional and the server makes me want to give him or her more). And here, when paying $2K tip for a $20K meal (and today in a just-decent place, my family pulls that kind of total), I think the waiter is pleasantly surprised and when we return, they always seem to remember us the next time.