I'm understanding you to say that you want the bottom of the maximized window to stop before it goes into the area where the dock is. In that case you want to "Reserve edge of screen" - OD Configuration, "Positioning" section on the left, "Reserve edge of screen" checkbox on the right side about a quarter of the way down on the window. That's for ObjectDock Plus, ObjectDock free may vary, don't know. Only available if the dock is not set to autohide.
Or maybe 2of3 got what you're looking for....