The key-like symbols next to the head with the 21% underneath show you the exact order in which to adjust the toilet pressure configuration. You need to try around a bit since one isn't the exact symbol you see on the toilet config, but it works after trying around a bit. And do the config in the order shown on the key-symbol, too.
It took me a while to figure that one out, too... so, I didn't memorize the exact levels and order of each since clever dev also makes the images switch back to the readied toilet once it's clicked in - as with all the other riddles. But it's easy enough to figure out with approximation, i.m.h.o. and a little fiddling. Also try to switch perspectives with the baseline on top and bottom, and after a couple of tries (less than 10) it worked for me...
Finally was able to progress. I clicked on the side of the toilet and the calibration screen popped up. And yeah, the game doesn't give the play any indication of where they should be click sometimes. Even just a little light appearing when you hovered over something interactive would have been appreciated. And the calibration puzzle took me a little but once I looked at the hint from a different angle, I literately facepalmed and felt so stupid. Finally was able to beat the game and although the toilet part frustrated me, it was a nice little puzzle game.