Pounds and psi are not the same instruction
A recipe that says press at fifty pounds is describing a force. A recipe that says press at five psi is describing a pressure. They only line up on one follower size, and the moment somebody makes the same cheese in a wider hoop the two instructions part company. Fifty pounds spread over a four inch follower is 3.98 psi; the same fifty pounds over an eight inch follower is 0.99. The wheel in the wider hoop is being pressed a quarter as hard while the make sheet says the same thing on both days.
This is why pressure is worth converting rather than copying. A pressure travels between presses and hoops; a weight does not.
The arm ratio, and which way round it goes
On the press in the form the pivot is 36 inches from the hanging point and 6 inches from where the arm bears on the follower. Long over short is six, so 70 lb hanging on the end arrives at the follower as 420 lb. Add the arm pressing under its own 8 lb at 20 inches from the pivot, which is another 26.67 lb at the follower, and the 3 lb follower itself: 449.67 lb of force on 44.18 square inches, or 10.18 psi.
Take those two distances the other way round and the ratio reads 0.167 instead of 6. The answer comes out thirty-six times off, and there is nothing on the screen to tell you, because 0.29 psi looks like a perfectly reasonable number for a soft cheese. Both distances come off the same pin with the same tape, and the long one is always the one you hang the weight from.
Backwards, which is how it usually gets used
Somebody hands you a schedule in psi. Ten psi on this follower needs 441.79 lb of force. Take off the 3 lb follower and the 26.67 lb the arm brings by itself, and 412.12 lb has to come through the arm — which at six to one means hanging 68.69 lb. That is a bucket you can actually fill, which is the point of a lever press.
Where it stops working is at the bottom end. If the arm and the follower between them already put more on the face than the schedule calls for, there is no weight to remove, and the page says so instead of printing a negative number. On a small wheel with a heavy arm that happens more often than people expect, and the fix is a lighter arm or a repositioned pivot rather than anything you can do on the day.
Half an inch of follower
Pressure falls off as one over the diameter squared, so the follower is a more sensitive number than the weight is. Going from 7.5 inches to 8 on the same 449.67 lb takes the face from 10.18 psi to 8.95 — a twelve percent drop for half an inch. Going the other way to 7 inches puts it at 11.68.
Nobody remeasures a follower, which is exactly why it is worth measuring once. If your pressing has never behaved the way a written schedule said it should, the follower is the first thing to put a tape across, before the weights and long before the recipe.
Questions people ask
How much weight do I need on my cheese press?
It depends on the pressure you are aiming at and on how wide your follower is, and this page will not name a pressure — that comes off the make sheet you are following. Put the pressure in the target field along with your follower size and the page gives the weight. On the press in the form, ten psi on a 7.5 inch follower through a six to one arm means hanging 68.69 lb.
How do I work out the lever ratio on a cheese press?
Measure from the pivot pin to where the weight hangs, and from the same pin to where the arm bears on the follower. Divide the first by the second. On the default press that is 36 divided by 6, so six to one, and 70 lb hanging becomes 420 lb at the follower. Getting those two distances the wrong way round inverts the whole answer and nothing on the page will look wrong, so measure both from the same pin.
Is the pressure calculated on the hoop or the follower?
The follower. It is what is touching the curd, so it is the area the force is spread over. Using the hoop area instead would understate the pressure whenever the follower is a loose fit. If the follower binds in the hoop the real pressure is lower again, because some of the force goes into the hoop wall, and that is not something any calculator can see from the outside.
Does the weight of the press arm matter?
On a small press, a lot. The arm in the form weighs 8 lb balanced 20 inches from the pivot, and through a six to one ratio that puts 26.67 lb on the follower before anything is hung — about six percent of the total, and the whole of it on a wheel being pressed lightly. It is why a heavy arm can make a low pressure unreachable, and why the page lists it separately rather than folding it in.
What pressure should I press at?
That is a question for the recipe you are working from and for somebody who knows the style, not for this page. Pressing pressure and pressing time belong to the cheese, the curd and the hoop, and nothing here suggests a figure or judges one. The fields hold placeholders so the arithmetic has something to chew on; replace them with the numbers off your own make sheet.