If I want a certain thickness in my final yarn, like DK, this is what I do ... first, decide how many plies I want (1, 2, 3, a cable?); let's say 2, that's typical. Then I can figure out roughly how thick my singles need to be ...
A DK (sportweight) is about 14 WPI (see wraps per inch chart at http://www.icanspin.com/WPI.htm).
The singles for a 2-ply are about 2/3 the thickness of the final yarn, so the single WPI is 14 * (3/2) or 21 WPI.
I pull out my handy-dandy (LOST! eek -- who sells these? please tell me! I need another...) VIP fibers measurement thing to get a mental picture of the thickness of 21 WPI. (There's also a chart in the back of Mabel Ross's The Essentials of Yarn Design for Handspinners.)
Then, I start spinning my singles, drafting out fine enough and measuring until I can be sure I'm getting about 21 WPI. At that point, I'll also take a length, fold it back on itself, and check the 2-ply WPI, to make any final adjustments -- do I need to spin the singles finer? thicker?
Once I've got myself "calibrated" (I know, this sounds like alot of sampling ... but in its own way, it's useful), I save off a length of freshly spun fiber, wrapped around a small piece of cardboard, with some unspun at one end so I have a picture of how much to draft, how thin to spin the singles, and a plied-back part at the other end of it to see what it looks like in the ply.
Then I refer to this card as I spin so I can keep my singles close to that sample as I go. And when I ply, I compare the amount of twist to that freshly twisted/self-plied sample.
This works for DK or any weight of yarn. Another thing to consider with spindles, is using a spindle that helps spin the type of yarn you want. For a DK weight yarn, I'd probably pick a spindle between 1 and 1.3 ounces; much lighter, and the spindle will stop spinning too quickly, much heavier, and the spindle will be pulling the fiber out of my hands before I can get enough twist in it to hold together.
I found a spindle weight to yarn thickness chart here, though it looks to me like she prefers a heavier spindle than me (grin). The Woodland Woolworks catalog has a chart with their numbers in it, you can see the PDF pages.
I hope this helped ... felt a bit long-winded to me (grin). The short answer might be, try drafting out finer if you find your yarn is coming out worsted weight. It may surprise you to find the fibers still hang together just fine, and you end up with a finer yarn. Remember, finer yarn needs a bit more twist, so be sure to set your spindle going faster or let it spin a bit more before you wind on.
(posted by me this day on spindlers)