Re: Indicator Placement 'Thingamabobber'
I agree with dhayden, you'd generally want your nymph to be right at the bottom. Fish often hang out in the slower current speed there because of the uneven contours of the bottom that give them a break from the force of the water, and of course if they're actively feeding by grubbing on the bottom for nymphs you'll want your fly to be where the food is.
The theory of needing extra line is that the current speed affects the length of line you'll need to reach bottom since the line with the fly and split shot doesn't go down in a straight line to the bottom from the indicator, but descends at an angle--
So if the water is 2 feet deep and moving, if you set your indicator 2 feet above the fly it might end up being only 1 foot or so below the surface. If on the other hand you set it 1.5x the depth, or 3 feet in this case, it will be closer to the bottom. 1.5x seems about the right ratio for many situations, but use less for slower currents, more for faster currents.
Again that's just the theory---
Hope this helps