I looked into this very quickly and generically in training mode. I might be wrong, but it seems the pop off from the ledge actually is generally based off of how high the character can jump if you let go of the ledge and immediately jump again. From a balance perspective, this actually makes sense to ensure that trumping a ledge doesn't just cause a character to fall the height of their second jump and essentially be forced to up B.
Essentially, it seems that the best you can do to guarantee more trumps into bairs is to just learn how far characters will pop from the ledge, and in what direction. Higher second jump characters will go up further, while lower second jump characters will fall off a bit more parallel to the stage. Vs another MegaMan for instance, you'll probably want to jump off the ledge, or detach and burn your second jump to get a little closer to "guaranteed", but I don't think you can consider it guaranteed ever.
On stages with solid walls (so basically just Omega forms of particular stages), you can trump into a SCAR jump and go for Flame Sword. Because of the really wide arc it covers, it's a guaranteed hit, and the wall jump height means you won't have to kill your double jump just for some mostly ineffectual damage. This makes me wonder if Sheik doing it consistently is confirmation bias or a function of Sheik's bair coming out with stronger frames faster than Slash Claw.
