TIL! Much thanks. Are you sure this (buy at best price in my favor) is how it is implemented commonly (e.g. the limit order is not just rejected for being an invalid liquidity maker order?)? I guess it is the Fill-or-Kill that is key; I suppose you can add an additional factor to that limit order to give yourself a range of acceptable prices (obviously you could program it, but is it a common order type on something like E-Trade?)?