BJ,
I think the best answer is to try to find a clear stretch of water, tye on a very visible nymph, and watch how the fly "swims" in the water. You want to experiment as much as possible under you understand how the placement of the weight affects the fly.
If you place the weight close to the fly, you will see the fly sink faster. The fly will also "swim" less.
If you place the weight away from the fly, the sink rate will decrease. The fly will also be able to twist and turn more as it is affected by the movement of the water.
What you want is to have the fly touch the bottom and "tick" along the bottom, but still look like something alive trying to keep aright and ready to grab hold of a rock or rise to the surface as it is carried along.
A "typical" distance then depends on the stream you fish. I "typically" don't need weight on Clarks creek. On Penns Creek I "typically" put several bits of lead (I use soft lead) starting 9 inches from the fly and going up the leader on the knots (I tie my leaders). Spreading out the weight helps with casting. With the weight dispersed, I do not have to chuck and duck as much. However, you can get some really horrible tangles if you throw the loop too tight.
A great book to read if you are interested in nymph fishing is On the Trout Stream with Joe Humphries.
Padraic