The biggest question in nymphing is: "How much weight should I use?". But there's no easy answer. Because the thing is, when fishing subsurface you can't see what's going on down there, but thankfully there methods to making informed estimations that keep your flies in the right place. Really it's not that difficult, how fast the water flows the heavier your fly needs to be to get down to the fish strike zone before the fly passes the fish. in shallow streams you may want your fly bouncing along the bottom of the river so it looks like a nymph that has been dislodged from its rock!