When I do floor press I usually keep my heel as close to my butt as possible and also flatten my lower back against the floor. Since the shoulder is prevented from going into a less than optimal position I see no point in using leg drive. Leg drive will only lead to an increased chance of becoming overextended in your lumbar. I will press my feet into the floor keeping the heel close to the butt but this is more for bracing/preventing myself from sliding around as opposed to using my legs to assist in getting the weight up. Another good trick is to think of pushing yourself away from the weight instead of pushing the weight up. Hope this helps!