In whitespace, after I get the portal glitch (which I barely even get) and fly to the red, I commonly find myself underneath the red. How could you guarantee not getting underneath the red?

My best guess would be that you're too small to move fast enough to get over to that section quickly. As your size changes, so does your horizontal speed, so the bigger you are the faster you will get there. To make yourself bigger whenever you perform the glitch, make the portal smaller. I recommend looking at the warpless route in the superliminal guide, found in resources. You can also use the picture that's attached there to get a more consistent setup for the glitch. Hope this helps!

You can also try installing the practice mod and enable unlimited render distance, which may help understanding what's going on.

I would also suggest you to join the discord server, as there are many people in there that will be able to answer your questions way faster than on these forums.