A 100% speed means that drag movement of 100 pixels will move 1 second on the timeline.
Your element starts at 126px and ends at 426px, so it moves 300 pixels in a duration of 1 second 22/30 frames (1.7333s).
300 / 1.7333s = 173.08 pixels per second
100 / 173.08 = 57.777, which means 58% is the value to use.
(trial and error to find this is sometimes faster than doing the math!)