I fish indicators quite a bit. I know there are some here who run the gamut from adamantly opposed to using indicators (bobbers in their words
) to only
using indicators. My feelings about it are, do what fits you and your style best. If you're catching more fish with indicators, great, if you prefer tight-line nymphing and 'feeling' the take, also great. I for one like to catch fish and feel that presentation and correct fly selection make the biggest difference. Because of my old eyes, nymphing under an indicator is my preferred method since I can still have the visual pleasure of observing the take.
Now, as for indicator attachment to your leader - the where's and why's - should always be determined by the depth and
speed of the water you are fishing. 1 1/2 times the depth works good for moderate flows of water 12-inches to 3 or 4 feet deep. 2 to even 2 1/2 times the water depth may be more appropriate for deeper runs along with swifter flows. The whole point is to get a good drag-free drift through whatever type of water you're fishing at the time and this will require some experimentation with where you place your strike indicator on the leader.
I usually start with what I feel is a bit long for the drift and then I work my way back down the leader until I get drifts that allow the flies to stroll along the bottom where normally the fish are holding. This method requires that you use a strike indicator that you can adjust as often as you need to (such as a Thingamabobber, etc). A lot also depends on how much weight or which weighted flies you may be using as this also affects the drift. The preferred result is that your offerings 'bump' a bit across the stream bottom. If you hang up on rocks or weeds occasionally then you've probably got it right. If you hang up every cast, you probably have too much weight or the indicator is too high on the leader, or, a combination of the two. If you're not hanging up at all, perhaps not enough weight or your indicator is too low on the leader - or, a combination of both. Experiment on the water you are fishing until it feels right.
Next problem is fly selection; pay close attention to what's going on when you get to the water you'll be fishing. Are fish rising? What type of rises? No rises, what time of year is it and what do the hatch charts tell you are the prevalent aquatic insects active for that time of year? Start nymphing with searching patterns such as a standard Pheasant-tail nymph or a Shop Vac nymph. Fishing a tailwater? Try a San Juan worm in different colors.
Now that you've got the drift right, the right flies, pay attention to your indicator at all times, don't become distracted, keep an eye on it. Any hesitation, pause, movement or some such indicates that something is happening down below and demands your attention - set the hook! Often it will be a rock or a weed, but, just as often, it will be a fish and your efforts will be rewarded.
Last of all, know that what you do for setup - depth, weight, fly selection will usually change and be different for every water you fish. It may even change for where you are fishing. I have a favorite stretch of the lower Henry's Fork that I fish quite often. I know how to set up for the regular stretches I fish, but, occasionally I will turn around and fish the water behind me, nearer the shore and it is much shallower, slower flowing and requires an entirely different setup than what I was fishing towards the middle of the river. This requires I change the setup.
I also fish a lot of slow-water spring creeks where the current is almost imperceptible and there my tactics and setup are much different than for regular river nymphing. The fish are big and spooky. The water is gin-clear and the fish react to the shadow of a bird flying overhead. I switch from using any type of commercially available strike indicator to using a special type emerger fly as my indicator - a Parasol Emerger fly. To this I'll affix a length of leader to my first trailing nymph, usually a bead-head mayfly nymph, trailed by another fly such as a scud or other some such fly representing the prevalent insects in the spring. The length of the leader from my indicator fly to the first nymph is only 1x the depth, meaning I don't allow for any leader exceptions due to flow or depth. It works.
Also, don't discount whenever you are fishing water from 12-inches to 3-feet in depth to use a bushy dry fly as your indicator trailed by a nymph matching the prevalent insect in the water that day - a typical Hopper/Dropper configuration.
A lot of options for you, but don't think that you have to be relegated to what has been written about 1 1/2x or 2x the depth of the water as this formula is only a guideline and not a requirement. Experiment on the particular water you are fishing that day and find what fits the circumstances you are experiencing. Most of all don't over think it. Take notes for use the next time you hit that water and remind yourself of what worked as well as what didn't work. Experience will be your best teacher with experimentation the thing that will lead you to finding the best solutions.
Problem is, the next time you go there whatever worked the first time may not work at all and you have to start back at square one again
But, that's all part of the fun of 'cracking the code' and the mystery and fun of flyfishing.
Most of all, have fun!