Yeah, 50 lbs is really on the light side for a cheddar of any reasonable size. Area of your cheese is: pi * r^2 (or radius * radius * 3.14159...) If we want Bob's 3.5 psi, we have to divide 50 / 3.5 = 14.2. Divide that by pi = 4.55. Then take the square root = 2.13. So at 50 lbs, your mold can only have a diameter of 4.25 inches to hit that 3.5 psi. Let's say your mold had a diameter of 6.5 inches, 3.25 * 3.25 * pi * 3.5 = 116 lbs of pressure needed.
This is why I make Caerphilly instead :-) (higher pH in the press, and therefore less pressure needed to knit the curds). And also, my mold is only 4.25 inches...
Also, agree that homogenised milk is the likely cause of the curd shattering problem. As Bob says, buying milk with the lowest fat content you can find and added pure cream to it helps a lot. When milk is homogenised, the fat globules get burst and the fat ends up coating the casein (protein) micelles (bundles). The makes it difficult for the rennet to do its work. By limiting the amount of homogenised fat in the milk, you limit the amount of fat that is coating the micelles. I've also found that mixing homogenised and non-homogenised milk works surprisingly well (60% non-homogenised produces decent curds, in my experience). Just a tip if non-homogenised milk is very expensive where you live.
