The various discussions of twists per inch and twist angle in forums led one overwhelmed reader to ask, "do you really measure twists per inch?"
I must say that for the most part, I just have a "ballpark" idea of what I want -- low twist singles, low-twist plied (which has more twist than low-twist singles...), firm 2-ply, or really tight 3-ply (sock yarn spinning -- been doing alot of that lately!)
I find the ply-back test (discussed in this post) helps me determine if I'm getting the yarn I want.
All that said, lately I've been on a twist-angle hunt, because of all the sock yarn spinning I've been doing. Measuring my own has been a bit after-the-fact, but it told me that my eyeball/ballpark attempt is pretty good. I think there's a fair amount of leeway in yarns in general, hitting an exact mark is mostly something for perfectonists (sic)
(based on a post by me to spindlers, 3/5/07)
If you're on a twist-angle hunt, too, then I can recommend these resources:
Mabel Ross's book The Essentials of Yarn Design for Handspinners (I know, I talk about this book alot -- it must be my favorite! yup!)
Eileen Hallman's video Spinning Cotton on the Charkha comes with a great glue-on-a-CD twist angle guide (please don't glue it on the DVD!)
Holly Shaltz's on-line tutorial Twist Angles
If you have another twist-angle resource, please post a comment to let me know or email me -- thanks!