My chair is at 18" (checked it prior to starting sts), and these are absolutely brutal.
The best I was able to do with these was to lift myself an inch or two - with my foot just resting on the floor, and no way all the way through these. By the end of the set, I was only tensing and pushing the muscle that should have lifted me up, but not actually going anywhere! I imagine that these will get better with practice and when the muscles grow and learn.
Each time I do one of these workouts, I wonder about how long rehearsals actually took. Cathe's crew must have practiced quite a bit on their own, prior to rehearsals, even. Everybody on screen is able to do these, but I'm sure they didn't come "naturally" - only with a lot of practice.