The formula I like to follow is a "team of three". I make each dropper by adding lengths of tippet to give between 12 and 18 inches between each fly. I use a four-turn surgeons knot or sliding barrel knot leaving the "downstream" tag end about 4 inches long. Once the fly is tied on, the dropper is about three inches. Any more and they are prone to tangle on themselves. To avoid tangles in general, smooth out your casting stroke and a make a much bigger loop than you normally would. Dropers and high speed chisel sharp loops don't go together so don't try for great distance. A roll cast is another good approach if you can cover the lies effectively. At the end of the day it comes down to practice. If you can practice off the water it will pay big dividends when you see big Brownies rolling on emergers 20 feet across from you.
When sight fishing, look over your shoulder from time to time, you never know who's behind you