The shadowcam is your friend. It gives you a preview of where the shadow will hit (directly behind what you see) and you can shape your primary light realtime by clicking on it (lightball on your interface. The one dead center) while looking through the shadowcamera (using mainlight parameter dials).

Those "legs" that do not cast shadows will become apparent why in shadowcam..

Right click workwindow>Camera>Shadowcam

 

Detail for the newbies here