that depends on who you believe, and what you are trying to accomplish.
But Peter Dixon believes that the whey pH should be 5.9-6.0 by the time the curd pack is formed and most of the whey is run off.
He further believes that the curds should be milled then the whey is pH 5.3-5.4
What drives this I am still figuring out.
I do know that some of my cheddars of mine that dropped into the pH 4.7 range were also dry and crumbly.
But there are other factors involved in drying out a cheddar:
Salt, Heat, Curd Dimentions, fat in the milk.
There are lots of theory there governing the effects of acidification, curd size, milling, cheddaring. All of these activities affect the structure, texture, and behavior of the cheese. Some of these may also be factors into why your cheddar is dry and crumbly...