I think you might be thinking of it sorta backwards? To determine the advantage necessary to land a given manual from an opener (X), you wanna take the advantage of the opener (Y) and add 5, so:
X=Y+5
The startup of your manual must be less than or equal to X-1 (remember that KI frame data doesn’t include the first active frame in it’s startup).
A real world Fulgore example:
L.Laser: Y=5, so (X-1) </=9
Any manual whose startup is 9 (becomes active on fr.10) will connect. For Fulgore, this is almost any button - you can even manual sweep at 8f:
L.Laser>cr.HKxHP>juggle
Incorrect, you just need to practice the timing. It’s sooner than it looks. In the case of LK Blade Dash:
L.Dash: Y=2, so (X-1) </=6, therefore:
L.Dash>LP manual>ender is true.
chg.L.Dash: Y=4, so (X-1) </=8, therefore:
chg.L.Dash>cr.HKxHP>juggle is true.
All strings were tested by hand to make sure the data wasn’t off, they all connect.