Best placement is really decided by your corals. Keep an eye on them, see if they like the flow they are receiving. If they look pissed, make adjustments, if your sand bed is blowing around, make some adjustments, if it's shooting water put of your tank, make adjustments.
+1 for adding another on the side. Placement is really subjective. I have 4 koralias, 750, 850, 1400, and a K3. I have 3 on the back wall facing forward, and the K3 on the side to create turbulance.