How deep are you fishing, and how fast is the water moving? Is it really slow-moving water (as in, painfully slow, long, drag-free drift- type tailwater)? If so, I would just go 1.5x the depth, maybe even just a hair less (only did this in Arkansas the few times I've fished there, or on the Guad in TX)
I was in SW CO a couple of weeks ago closing down my folks' place for the winter, and fishing there this time of year was the exact opposite of how I would fish in, say, the Little Red in Arkansas, the Guad here or the Blue in OK (Blue being the closest trout fishery to my current locale).
William is pretty spot-on with that advice, and variations come with specific applications. I might throw an unweighted nymph about a foot behind some tung shot (I quit lead and felt) and another off of that one . If your shot gets hung up on the bottom, try rolling some tung putty on your line.
I like weighted nymphs in slower stuff, a little more of a direct connection. I don't feel like it matters as much in faster water and use shot more often, unless its a dropper, in which case I almost always use a beaded or weighted nymph (unless its a trailing emerger), though it just depends on which flies I want to use.
I guess there's just no short answer, just like everything else in our sport... Just listen to William, apparently I'm not good at the whole brevity thing
