That's a great press, Tim. The leverage is impressive. A 5 gallon bucket filled with water is about 40 pounds. As you may know, you multiply that by the ratio of the two lever segments (from piston to weight, and from pivot to piston) to get the effective pressure on the piston. If the piston is in the middle (half-way between the pivot bolt and the weight, that is 1/1) then the pressure is 40 pounds. Moving the piston and pressing box closer to the pivot increases the pressure. So if the piston is located 1/3 of the length of the beam from the pivot (2/3 of the length from the weight), you get a a ratio of 2/1, and you double the effective weight to 80 pounds.
Divide that by the surface area (length x width in inches) of the top of the pressing box to compute the psi (useful for Perique pressing, in which you aim for ~40 psi on the tobacco). These calculations ignore the weight of the beam itself, which in your case is significant with a 4" x 4".
My cheese press can hang a total of 1-3/4 gallons of water (~14 pounds), and generates about 105 pounds with a piston position closest to the pivot. I've found that a mere 2 psi works perfectly for simple leaf pressing.
Bob