At BaB +20 you could take -20 to knock them back 100'. It gets +5' distance for each -1 you reduced your attack. By that time, I really don't think a melee character throwing someone 100' is much of an issue. If you went 5' for each -5, it would only start being remotely useful at BaB of +10 or better. Sure, you could knock them into the floor (down is a direction ), but you would be better off just using Power Attack.