If you want my honest answer, play around with it lmao. Find a time that fits for the first part (rocking back), that’ll be the first time. Then I’d figure out if there’s any kind of pause between the transition from backwards to forwards, and if so how long (even if a very very short time). That’d be the second thing. The third thing would be figuring out the time it takes to return back to the original position. Once all 3 values are figured out, you can loop the overlay.
I know it’s very tedious, but I feel it’s the best way—as I wouldn’t have the patience to time an animation. It could also vary depending on devices 