You will all fear Sheik ftilt, prepare yourselves as the Brawl Sheik mains are coming for you.
Good find, it's only a matter of time now.
Edit:
Damage Decay on Mario fireball
Fresh, 0 Decay 5 damage 4 to 9 (Mario started at 4%)
1 Decay 5 damage 9 to 14
2 Decay 4 damage 14 to 18
3 Decay 4 damage 18 to 22
4 Decay 3 damage 0 to 3
5 Decay 3 damage 3 to 6
6 Decay 3 damage 6 to 9
7 Decay 3 damage 9 to 12
8 Decay 3 damage 12 to 15
9 Decay 2 damage 22 to 24
This shows there are still at least 10 'levels' of decay which mean decay could work in a similar method as it did in brawl HOPEFULLY the exact same way. What is interesting though is how after you 'refresh' your move you did 6 damage. You mention it could be because you used fireball close or that moves could do half percentages (something I thought could happen in brawl but I never saw confirmed) which is a great insight. You earn my respect for your dedication and quick mind.
Edit2:
Knock back is going to be much harder to test constantly without a friend or two but I suggest using mario usmash if you do decide to test it. The method of testing is pretty simple, do some of those crazy smash4 mario combos until you reach your desire percentage and then attempt to kill with usmash.If it works take a note and kill yourself, then do it again and again until it doesn't work anymore. Once you found a percentage that it doesn't work try your best to get to the half way mark between the two 10% percentages example 80-90 so 85%. Once you learn if it can kill or not at that percentage you have to start using grab pummels to raise percentage. Grab pummels are also effected by decay so they are a easy way to increase damage 1% at a time. Sooner or later you will find the exact percentage that Mario usmash will kill without DI after that you have 2 options.
Option 1: You learn how much damage a fresh mario usmash will do, raise your punching bag percentage to where fresh usmash damage can used to get to your goal percentage then try the slightly decay usmash to kill. I don't like this method personally
Option 2: You get another friend and have him wait around and you only use him to decay usmash.
edit3: Oh sweet we got a real researcher playing the demo! Subscribed, thank you for all your hard work <3
