best LOS to achieve full missile fire

[Jagger2002]

Ok, I just had a situation which has happened many times in the past and I still don't know how to determine best LOS to a tgt. I moved a light cavalry unit with crossbow 2 squares from a target and needed to change direction to get a LOS to the target giving full fire. I could chose two different squares to face. Unfortunately, I couldn't determine which of the two hexes I should face. If I were to move to the target square, the movement cost was the same going through either adjacent square-10AP. Movement distance was exactly the same to reach the target square. So I just chose one square to face and of course, got half fire instead of full fire. How does everyone determine which square to face to get full fire when the target is not on a direct line from the firing unit? I haven't been able to get it consistently right just be eyeballing the situation.