I don't know if this has already been mentioned, and I don't want to go through this entire thread, but you can extend the percent window of Dair > Up-B by doing a jump then using Up-B in the air. You could do it from a wide range of percents by mixing short, full, and double jumps, but I think the most optimal is before Up-air starts to reliably KO(and it gets pretty tricky at higher percents). So:
Light
45-70
Medium
65-90
Heavy
85-120
tested on Pichu, Mario, and Bowser and these should be taken as rough estimates, as I don't have any way to test DI and character size and weight, even among the same weight class, can make a difference. The easiest way to do this would be to have tap jump on and buffer a jump with the stick and press B when you get to your opponent, while still holding the stick. That should allow you to observe what angle you're going to hit.
You'll need to hit around the top of the first hitbox and angle the second, as it will pop them at an angle. You can see which angle it's going to pop them, by looking at which side of the first hitbox you hit them with. So if you hit them with the left side, it well pop them left and you will need to angle left. That goes for hitting with Zelda's head as well.
I think this will be very helpful on small characters, which are notoriously difficult to hit with options like LK and might not be safe to hit with Up-B OoS when being shield pressured. So say Pichu is F-Tilting your shield, you can Dair OoS and take his stock around those percents. This should discourage from trying the same in the future.