It doesn't seem as complex as you made it out to be....
(Disclaimer: I am still learning about decay, but to the best of my knowlege...)
It's obviously not that simple.
Kirby is used as an example of a floaty balloon type character. All characters are individual, even the clones, and as a result (though sometimes the methods may be similar by sheer coincidence) each individual matchup will display different reactions to the f-tilt. This means you will need to get f-tilt to medium decay for certain characters, high decay for others, and light decay for the rest (i.e., kirby).
Logically, this is my interpretation. Please correct me if I'm wrong.
P.s., AL... Instead of flaming and shunning the newcomer...
Watching vids and reading about techniques will never get you to where you (presumably) want to be. Sheik's techs, and from my experience most of the cast's techs, are for show. They're glitches and by-products of brawl's obscure physics that cause pretty things to happen. Nothing more. Occasionally, however, a useful (sometimes essential) tech comes along that will serious benefit you. Sheik, though, has no such
essential techs. Helpful, yes, required, NOOO.
Playing experienced players is the only way to get seriously better. The ONLY way. Watching videos and reading guides will give you the knowledge, but it's up to you to get the application right. All Sheiks are different. You need to find your own style and tweak it -- this is done by crits of your matches and first hand advice from experienced players. Eventually you'll come to develop a style, or rather, an understanding of Sheik and Brawl fundamentals. Then you'll be an good player.
I can offer you online matches and give you what I have to offer, but I not by any means "experienced" to the extent of the players I was referring to.
Not to be hypocritical and contradictive, but I would advise you to watch videos of TristanWin, Gimpyfish, Ankoku, and RyokoYaksa. (I can think of others but I don't know their Sheiks all too well, and I am sure the ones I named will not be happy about this anyway.) Even though they are videos, there are things to be learned.
Gimpyfish can show you the fundamentals. Chaining b-air and tether gimps.
TristanWin can show you how NOT to be predictable. Brawl is about living in the moment, so to speak.
Ankoku and Ryoko and show you how to win a match. Spacing and f-tilt (granted I have not seen much of their work recently.)
Obviously, playing someone in person will teach you far more than any video, but most Sheiks tend to dislike WiFi and most Sheiks tend to dislike newcomers (often with good reason).
Go to tournaments if they're available to you. You don't need dittos to teach you how to play Sheik. And so ends my almighty lecture/pep-talk.
Once again, I am not an experienced Sheik, but from what I've learned I like to think I have some idea of what I'm talking about.