I keep failing getting the berry for 400a. How does it work?

The berry's hitbox moves up and down with it, so you need to dash for it when it's at the bottom of its cycle. If you notice in other runs, they would waste a few frames before jumping- this is to wait for its hitbox to be at a reachable position. :)

It should be noted however that unless you're doing 100%, left side 400 is only faster if you get the corner jump exit.