Mary, Carter is correct in that the springs are going to give you just an approximation of the real force applied. That being said they get you in the ball park and can still make great cheese with them. If all works out then you can invest in something better and more accurate.
Also, be aware that the pressing weights typical of the 'recipe books' are based on a certain mold size. There has been much dicussion here regarding translating those back to actual pounds per square inch (psi) a.k.a. pressure. The idea is that you want to use the same pressure to press your cheese independant of the mold size. Carter, has some charts and such that help with the calculations but basically if you need 10 lbs of force applied to a 4 inch mold then you'd need 50 lbs of force on a 8 inch mold to maintain the same pressure on the whole surface of the cheese. This is definately the cliff note version...
I'm sure a quick search will find the original topics.