Multiply your hanging weight by the arm length ratio.
Say the distance from the stamp to the fulcrum is 5" (A ) and the distance from the fulcrum to your hanging weight is 25" (B), then B/A = 5. Multiply your weight by 5.
So if you hang a 6# weight, your press is at about 30#.
And while it helps to have a general idea of the weight used, most often it is best to adjust weight based off observation of the cheese. Don't follow instructions that specify a general weight without taking into account the diameter of the mold.
The goal is to start off lightly so the cheese can gradually lose whey, then increase the weight incrementally so that you maintain a steady seeping of whey for the first hour or so. Also really depends on the type of cheese and how dry the curds are already. Press too firmly to start and you will essentially trap a bunch of whey within the curd because the rind has been overly pressed and whey cannot escape.
The exception would be cheeses/curds that are already salted, like a true milled curd cheddar. Press heavily from the beginning.