Its a little more complicated than that (when trading currency futures - if your not day trading)....you have to take into account the currencys 'cost of carry', its time to expiration, and how that is priced into the futures contract. Otherwise you could essentially give away all your profits (or increase your losses) if it cant move enough to cover that carry and time to expiration that is priced into the futures contract (and the divergence to the spot/cash price up to expiration).