I remember when I first started fly fishing thirty-five or so years ago. I fished bluegills at Pine Lake State Park and at the local pond called Three Mile (because it was three miles out of town). I had a telescoping tubular steel rod, an automatic reel, level 6 wt line (I have no idea what weight the rod was) and used one of those metal pins to attach a six foot piece of eight pound mono on as a leader. It cast so bad it's a miracle I still flyfish. I did catch a lot of fish and I did have a good time.

When I got back into fly fishing ( I had been bait fishing with a fly rod for years), I learned about tapered leaders. Even read up on Harvey leaders, but thought it would be a drag tying all those knots. I did find that the bait shacks carried a line of knotholes tapered leaders and used them. I was not very happy with them though. Then I read Gary Borger's Nymphing. There he explained his UNI system. Less knots. I tried it and I liked it.

The basic problem with purchased leaders is that the cheap ones are one size fits all. Weather you have a 3 wt. or an 8 wt. One size.
Note: The butt of the leader should be roughly 2/3 the size of the end of your fly line. To satisfy my needs I made the following chart for the lines I was using at the time. Note that One butt size does fit a lot of lines, but not all. It illustrates Borger's concept. Don't just use this chart. Check out your line. Pay attention to how stiff the leader material you use is. Modify them to get what you need. Gary's son Jason does just that. In one of his articles "Take Me To Your Leader" he shows some expansion on this concept.