Think of your press beam as a class II lever. One end is stationary and that's your fulcrum, and your point of pressure instead of being at the other end is somewhere in the middle, the closer to the fulcrum you put it the more mechanical advantage you get.
So if you have your pressure in the middle, you have a mechanical advantage that yields a pressing force equal to 2x the weight of your beam. If you have your pressure 1/3 of the way from the fulcrum to the far end, you will have a pressure equal to 3 times the weight of the beam, and so on.
But really in the end, you still need to have a very big timber to achieve a good pressure and get your pulp good and dry.
