Ya, start with a 9-foot tapered leader and go from there. There's all kinds of leaders that work well for a given situation, with the possible exception of a 9-foot piece of straight line.
Tapered leaders help you present
your fly. I often fish leaders of 12-18 feet with dry flies to achieve long, drag-free drifts to "spooky" or "selective" trout. Those are not needed for other places, like faster moving free stone streams and rivers. For bass fishing in murkier water and around cover, 6 feet might be plenty. Same for fishing wets/streamers on a sinking line. Use a basic leader, and with some trial and error you will find what works best for you, your style, the waters you fish, and the fish you catch.