Example: Buy $100 worth of bitcoins at $13 ...if the trade fee is 1%, then the amount of bitcoins for the order would be $99/$13 or 7.6153 bitcoins (truncated to 4 decimal places).  If the current low ask is $14, the then order is placed as "buy 7.6153 bitcoins @$13" ...if the current low ask is $12 and can cover my entire order, then it will execute a trade immediately, buying $100 worth of bitcoins (or $99/$12 = 8.2500 bitcoins).

This is by far the most common trade I do.  I rarely sell bitcoins on the exchanges (instead, I like to spend or save them to spend later).  And when I buy, it's usually for the entire amount that I have on deposit with the exchange (because I have no desire to keep a balance of anything (BTC or USD) in the exchanges).