I'd call the OEM for a manual on this. Seems to me there would be a graph or chart on the machine to relate the bar reading to the cylinder bores or effective piston areas to achieve a certain tonnes/m set by a potiometer scale. It may benfit you to add a digital panel meter off a pressure transducer so this reads directly. It won't be exact unless you placed strain gages on the housing and averaged their outputs. A whole lot more calibration there though.
No display or tonnage readout on the press? Does it reverse on pressure or ram position?
Here in the States it is tons/ft since the dies are rated in tons/ft and the material bend tonnage rated likewise.
Regardless, you could start your bend on low pressure and raise it until you can achieve a decent bend. The potentiometer could then be marked with a scale relation to the tonnage close enough. That only applies if you are air bending and using a lower V-die standard of 8 x material thickness. Any deviation to that standard will alter the actual tonnage required according to your bend chart. The tonnage on the pot setting will remain the same, but it will take more pressure for narrow dies and less pressure for wider dies.
If your are coining or bottoming the dies, be very careful not to exceed the die load rating. Sneaking up on it is much safer, but knowing their limitations is self preservation.