Most advice I've heard and read about optimum cleat placement is to position it in such a way that the ball of your feet sits directly above the center of the pedal spindle.
If the above does not work for you and experience some discomfort, try to position them were it is most comfortable for you and therefore as pointed out, thru trial & error. A quick way to do this is to mount the cleats loosely. Mount your bike on a trainer and then pedal lightly. With the cleats loose enough to move, try to move your feet on the pedal while pedaling until you think you're comfortable. then stop pedaling and with you still on the trainer and cleats on the pedal, hold that staionary position and ask someone to mark the cleat position. Naturally when u un-clik the cleats may move. Just re position them as per the mark.