Usually in most circumstances when fishing small dries you throw a 9-12 ft leader. Some instances call for a longer leader, but to be honest not many people including myself can have much accuracy and turn a fly over with a leader longer than 12 feet. If you have a targeted fish actively feeding, you want to position yourself in the best possible angle to get a drag free drift into his feeding lane.
I recently read an article on dry fly fishing where they asked "How far would you go for a single peanut". That analogy ran true when compared to dry fly fishing. Would you get up in front of the TV to go into the backyard for a single nut? No way, but I would probably reach into the bowl on the coffee table right in front of me for snack. You must get your fly in front a fish to even have a chance at success. A trout will only expend a certain amount of energy to eat a small morsel. That is why all fishing guides love fishing grass hoppers as fish will move feet and not inches to take a piece of prime nutrition. If you are fishing a size 18 BWO, a trout is not going to go out of his way to chase it down. There are probably plenty of naturals coming downstream and he will take the easy path and just wait for the one within 6-10 inches of his nose and enjoy his meal without a lot of work.
Gauging distance is one of the most important details in dry fly fishing. I always work out my false cast to one side of the riser aiming to drop my fly at least 3-5 feet above the trout. This insures I won't put my colored fly line over them "lining them" and spooking them or putting them down. A trout must see his dinner coming to him. Throw it too far above him and you risk dragging your fly over him, throw it too close to him and you will spook him, throw it behind him and you have zero chance.
One other major thing to keep in mind is when you are false casting you are throwing tiny droplets of water off your line. You have to be careful not to rain on your trout. Throwing unexpected rain droplets on your target may sink your ship before it sails. Always false cast to dry your fly off or to work out the appropriate distance away from your target. Upstream, to the left or right works as well. If you have a pod (many fish) working, error on the side of caution. But remember presentation is EVERYTHING. A natural dead drift catches more fish on average than being able to cast 70 feet and throw a double haul.