Downback Zeb Skip can have consistency problems caused by Samus's subpixel positioning. Both horizontal and vertical subpixel values can cause the skip to fail.

We can normalize Samus's horizontal subpixel value by bonking the T-shaped platform during our jump, since bonking a wall sets Samus's horizontal subpixel value to 0. (The Zeb itself doesn't count as a wall; it's a solid enemy.) As long as you bonk this platform, and you're NOT pressing left on the D-pad at the moment you contact the Rinka, you will be guaranteed a good horizontal subpixel for the zeb skip.

You can use the "X Position" infohud mode to see if you're doing the platform bonk correctly. As in this video, you should see Samus's X position stop at 837.8000 on the way up; then 837.0000 on the way down. As long as you see these two values, your horizontal positioning was correct for the zeb skip. If you still fail the skip, then either there was a problem was with your vertical subpixels, or you messed up an input. After you you successfully clip into the zeb, your horizontal position will become 836.0000.

Samus's vertical subpixel positioning depends on the frame on which you start your downback (specifically, it depends on how many frames you are in contact with the Zeb during your fall, since horizontally colliding with a solid enemy resets your *vertical* subpixel). After the top Rinka knocks you back, there is a pattern of "good frames" and "bad frames" where starting a downback will or will not give you the correct positioning. The pattern is: 4 good frames, then 1 bad frame, then 5 good, 2 bad, 2 good, 5 bad, 1 good, then all the rest are bad. Clearly, it's best to start your downback relatively early to aim for one of the clusters of 4 or 5 good frames.

The first bad frame is this frame: https://wiki.supermetroid.run/File:DownbackZebBad0.png
The next cluster of 2 bad frames starts with this frame: https://wiki.supermetroid.run/File:DownbackZebBad12.png
The last cluster of 5 bad frames starts with this frame: https://wiki.supermetroid.run/File:DownbackZebBad34567.png