It is also not linear. If it were linear it would be as simple as move the handle a known distance and record how far the ram moved and you will have a ratio. This is constantly changing though.Force developed by a standard operator is just a standard input multiplied by the mechanical advantage of the press. This is the force developed in the last quarter inch or so of ram travel. Any engineer can do this with a pencil and a calculator.
More difficult when almost none of the travel matters except the last few thousandths, where the work occurs. Lots of presses have “cam over” or the linkage goes over center at this point. That and manufacturing tolerances in the linkages even compound the difficulty/accuracy of any such measurements.