Select the camera you want to render with.  Find the focus distance parameter dial.  Adjusting it causes crosshairs to appear in viewport.  Centre of crosshair is approximate size of a typical figure's head.  Adjusting the focus causes the centre to move to/from the viewplane.  You should be able to visually judge where your focal point is as you calibrate.  It works very well for me (except for one time it seemed to detach from its camera).  Hope that helps...