You'll have to capture the pressure under the ram piston. I believe, from the drawing, that you will need to drill through the base into the void under the ram, and avoid the reservoir. Might have to drill at an angle.
Pressure calculation is per area of ram piston. you will need a measurement of the ram while you have it apart to drill. I trust this gauge has enough range? 50 tons is 100,000 lbs, divided by the area of the piston.