I read a couple strategy articles that discuss food counting in the fat cross of your city. It stated to count any food over 2 as positive and under 2 as negative. It said that if you end up with -3F, for example, then build 3 farms and you're good on food for that town. Wouldn't this be overkill though? Farms grow to provide more than the extra 3F correct? I would think that if you built 3 farms you would definitely come out with more than you need.

Yeah that is before biology. After biology you can change some farms to workshops or watermills if you want, or windmills to mines.

You don't need to work all the tiles, farms are good for working more mines or whipping or sometimes even specialists(mostly you want to work specialists just of food specials though), but they won't really allow you to work more cottages than you would otherwise(but growing on some farms can give you more benefit out of a city, but you should later convert those farms into cottages).