I gauge the singles for my 2-ply at 1.5 x the wraps per inch (WPI) of the ply -- so for a 20 WPI 2-ply, I want a 30 WPI single.
And, there are 25.4 MM marks per inch, so I know I need my single to be slightly smaller than 1 MM, to squeeze 30 of them into the inch...
(as opposed to using, say, the 1/16 marks on my ruler as a guide and getting 2 singles within one, for 32 WPI...)
For a 3-ply, I aim at having my singles be twice the WPI of my final yarn -- so for a 20 WPI 3-ply, singles need to be 40 WPI.
Once I figure out what I need, I draft and spin and sample a bit until I get a clear picture of how finely I need to draft it out, and then I take a mental picture and also a physical sample of newly spun single wrapped around a small piece of cardboard, with the end fluffed out so I can see how much fiber is in it.
The card is also a useful place to note the fiber type, source, and the date I started spinning it ... useful for those extended, 3-year projects!
(based on a post by me on spindlitis, 19May07)