Thanks for the replies. The common answer I get is, "if you need to ask, you shouldn't be building one!"
However, I did get an answer from an engineer and it's not rocket science.
So I have stopped asking now but I did ask a lot of people who have built presses for themselves and for sale to the public! The remaining question I have is did these people just guess? I know some of them did because, by their own admission, they had to modify their design after the frame flexed on trial run. What is really amusing, one of these people gave me the "if you need to ask..." reply.