Cosmic is correct. Raw tobacco leaf often gets stored in for years at a time and is bone dry before blending. It doesn't hurt the leaf to be that dry.Most tobaccos have been dried to almost crunchy several times before it gets to the blenders. It can be kept at bone dry until the cells start to break and release the natural essential oils of the leaf. Once the oils are gone, I'd say that it has reached the "Too Dry" stage.
As to where I like to dry mine to... depends on the blend. Plus, I don't always do the same thing each and every time I smoke. So, you just have to try drying for each of the blends that you like, till you start to develop a sense of where your preferred levels are, and you'll start to anticipate on new blends. It is more Art and Less Science when it comes to preferences.
On the other hand, once it's cased and blended and given a topping that's a different story. Aromatics and blends with a lot of topping are rather sensitive. The first thing that evaporates is the topping when you dry out a blended tobacco. Take a tin of Haddos Delight for example. Dry it out and the distinctive tin note and plum flavor disapear and what you're left with is pretty much just a standard vaper. It still smokes fine and tastes pretty good, but it no longer tastes like Haddos.
I tend to prefer my tobaccos bone dry, but I dont smoke many aros or semi-aros... except War Horse, which I think can tolerate being dry and still taste like War Horse