You can really get into this...

Lots of variables. If you really care to break it down, you can find advantages to using different butt diameters and leader lengths based on:

The fly you fish. Size, type (streamer, nymph, dry...etc.).

The type of fishing you intend to do. Surface, subsurface (upstream, down, dead drift or animated.)

The conditions you might face. Water clarity, water depth, current speed, stillwater, wind velocity and direction.

The type fly line you are using. Floating, sinking (different sink rates?), sink tips.

The type of fish you are fishing for. Trouts, bass, salt or freshwater, toothy critters, etc..

I'm sure I missed many of the options out here, but I'd say it would be safe to say that I could come up with well over 1,000 leader butt diameter/leader length combinations and be able to justify a reason for using each of them under certain circumstances...and that would be WITHOUT considering tippet strength...And for EVERY one that I would consider 'ideal' for a certain application there would be at least fifty people that post here regularly that would use something different under the same conditions.

In any event, there is no 'correct' answer to this, only that if it 'works' for you, then you have it 'right' for YOU.

I really beleive that a 6 foot piece of 4 pound mono would work just fine for 99% of the trout fishing done in the world...but I really like tinkering so I don't keep it that simple.....

Good Luck!

Buddy