I did some research on this earlier today, you can tell if the zip is possible based on the brown line at the top of the screen.

I have screenshots I can post eventually, but basically it goes

Thick brown line: Zip is impossible (I checked every possible frame to hit left on in frame advance, it can't be done) Thin brown line: Zip is a 2 frame window. There seem to be multiple levels of thinness so it may sometimes be more or less lenient, but in general the thinner the line the better. No brown line: You don't even need to press left to zip, the game will zip for you. It's actually fastest here to not hold right during the zip and to press no inputs until you've zipped, then hold left. Alternatively you can slide left at the top which seems to be slightly faster as you exit the zip in the cavern below while already sliding.

As of right now I can't find any setups for specific brown lines