Taking on the 'throw in $20'

Maybe I'm overly anal, maybe I am just better at math or maybe I realize how hard waiters work but it's shocking how difficult it is to do figure out a check including tax and tip in a large crowd. First of all nobody knows how many people to divide the total by so trying to get a headcount is nearly impossible as people are standing up or in the men's room so at some point somebody says '$20 each' because everybody wants an easy number but somehow it never works out which is weird because with a prefix it should not that complicated.
$14.95x0.0875x1.20->$19.50 so if theoretically everybody throws in $20 we should be well covered but as always somebody out of the 20 orders another drink, substitutes egg-whites or gets a latte instead of a normal coffee and now it comes out to $20.62 a person and nobody has any singles
So although I hate people who will analyze the bill and go dutch my new rule is if you have a prefix and you get something different you are on the hook for a 50% premium if only for convenience.