How do you know if your nymphs are making it down to the strike zone along the bottom? Why not just take a look? Try dropping your flies in the river where you can see them sink and adjust from there.
To assess how your flies will sink in the seam you are planning on nymphing, make a very short, close-in cast upstream in clear water that will drift only two or three feet past your legs. You may only need to lift your rod tip up and drop your flies into the river upstream. The goal is to place the flies in the river close enough that you can observe and assess the rate at which they are sinking.
From there, everything is a comparison of the depth and speed of the water that you observed the flies sink and the depth and speed of the water you are going to be fishing.
If your fly is dropping nicely in slow, shallow water, but you plan on trying to reach fish along the bottom of fast or deep water, then you know you need a heavier fly or some split shot to get the fly down. Even more for fast AND deep water. On the on hand, if you are fishing water similar in depth and speed to the water you watched the flies sink, then you might have just the right amount of weight.
In the same scenario, if you observed your fly sinking to the bottom immediately, that may mean too much weight for fishing nymphs in similar slow, shallow water. However, you may have just the right amount of weight if you are planning to catch fish along the bottom of deep, fast water.
It is as simple as watching the flies sink in the current and then deciding if you need more or less weight for the water you are nymphing. It’s a wag, but it helps.
The next time you are not sure if your fly is getting down, Wadeoutthere and take a peek.