First thing you need to do is determine what the flow stress of the stainless steel is at the minimum temperature you will forge at.. this is the stress the material will flow at and is not the yield or uts. It’s not all that easy to find.
Next is to calculate the total projected area for the finished part including the flash. You don’t have to worry about surface area, just projected as if it were a 2D drawing.
Multiply the two and you have your min force required. Divide by 2000 for tonnage. This will be the absolute minimum. Throw in a safety factor and maybe som room to grow and you are done. Easy-peasy.