The physics
A dropped ruler starts from rest. It speeds up by g every second. The distance it falls grows with the square of the time.
d = ½ g t² v = g t v² = 2 g d
So the mark you catch gives the time you took: t = √(2d / g). Put d in metres.
From rest, the distances in equal time steps go 1 : 3 : 5 : 7 …