Depending on the type of the press, their rated tonnage is calculated differently:
Hydraulic Presses usually have 2 or 3 tonnages related to the ram velocity. The tonnage is true throughout the pressing portion of the stroke.
Mechanical Presses are usually rated for their tonnage one half inch off the bottom of the stroke. Above this half inch, the tonnage diminishes quite rapidly.
Link Drive Presses are an exception. More of their tonnage is available higher up in the stroke.