I think it is not so much a matter of the Angle Start and Angle End, as it is a matter of the angle of incidence between the cord of the light and the floor, the actual light object should point downwards to some degree. In any case, if you can't see a shadow without the fill light from above you will never see one with a fill light. Any extra light shining on the floor can only attenuate the shadow. The only way to make the floor lighter without attenuating the shadow is to lighten the Object Color (if it is not already white) or use a lighter texture map.