I don't believe the float is a quantity that is calculated. It is by definition, the shares available currently for trade. This is diffent than the shares outstanding, as the company may own shares of its own stock, amonst other factors. Typically, a large float yields more liquidity, and vice versa. I would not charaterize a float as good or bad at either extreme; each has its own advantages and disadvantes. Typically small floats are more easily manipulated, like ssnc. A large short order taken out on a small float will send the stock tumbling. But likewise, a large buy order in a small float will send the stock to moon (like ssnc on friday).
So, in short, a good float is one made of root beer and vanilla ice cream.