How thickly should I spin a given breed of wool?

Anne Field's Spinning Wool: Beyond the Basics (currently out of print) discusses spinning to the "crimp" of the fiber -- that your yarn's twists per inch would match the crimps-per-inch of the fiber. The basic idea is, the higher the crimp, the more twist you'd put in it. She also ties this into wraps per inch.

Her basic formula is: You want the final twists per inch (TPI) to equal the crimps-per-inch of the wool, and the final wraps per inch (WPI) to be twice the twists per inch (WPI = 2*TPI). Anne Field is mainly discussing doing this for a balanced 2-ply yarn. The single would have a higher TPI and WPI than the plied yarn, in a 2-ply -- that's why I say "final".

_However_ ... I believe Anne Field is aiming at spinning for knitting yarns, i.e., open, bouncy, most elastic yarn you can get from a given fiber. She is also careful to point out that this isn't a hard-and-fast rule; there are times when you want higher twist, want to not "match" TPI and WPI, and just want to make the fiber do what you want.

For example, most needlepoint wools are coarse wools (Wensleydale makes lovely needlepoint wool, for example) -- given its crimps per inch, you'd "want" to spin it loosely and very thick. But for needlepoint, you need to spin it fairly tight and fine. And, the wool will let you do that, and give you a great result besides.