I've set up a few of these at my local bar and at the legion. I've never seen anyone have a set number of squares that must be sold, the establishment has always taken all the squares not sold by gametime. If you're running one privately, it's probably a good idea, unless you want to gamble large.
As far as payout goes, 50% for final score, 25% for halftime and 12.5% for first and 3rd quarter is pretty standard (assuming you're talking football). Some people add a reverse final score but I've never bothered with that.
