Pat has it right on the money. Personally I typically say 50% as well, but to put it in perspective, it is "Primary Source." If you make 40% of income from poured drinks, 30% from cheese sticks, and 30% from varous other goodies, in my opinion the poured drinks is still the primary source of income. I love cheese sticks more then the next guy, but they are clearly not the primary source of income, nor is poured drinks 51% of the income. I would still presume in a common sense format that a "Bar" or "Tavern" would clearly make most income from poured drinks, although perhaps that isn't true and some day we will need to find out. Perhaps the majority of most bars make their primary income from beer, honestly I don't spend enough time in bars and have always just assumed what we all mostly have assumed regarding the poured drinks being the primary source of income at all bars.
Perhaps in reality, we can conceal in every bar except the ones who don't serve beer, although I doubt that is the case, nor the intent of the law in question.
-jason r.