How big around is the main part of the bottle jack body? If you know the body diameter, you can subtract a little for the wall thickness and probably get pretty close to the approximate hydraulic piston diameter. Once you know the piston diameter, it is pretty easy to figure the pressure required to deliver the rated 20TON force. As an example, a 4" cylinder has 12.57 SQ/IN of surface area. If you divide that surface area into the jack output force of 20Ton/40,000 LB, you get a hydraulic pressure of around 3182 PSI to provide that much force. A 6" piston has a surface area of 28.27 SQ/IN and would only require 1414 PSI of hydraulic pressure to deliver the same 20TON force.
Once you dig into the valve, you may also find a safety pressure relief valve associated with the valve mechanism that prevents the maximum pressure/jack force from being exceeded while pumping. It would probably be a good idea to duplicate this relief circuit in some fashion to keep the bottle jack or press frame structure from being damaged if your hydraulic source inadvertently delivers too much pressure to the system.