The maths

A pendulum's full period (out and back) depends only on its length and gravity, not on the weight or how far it swings, as long as the swing is small:

A tick, one swing from one side to the other, is half of that, so one tick = π√(L/g). The side-to-side time is exactly half the full period.

For a tick on every beat, set one swing equal to 60 / BPM seconds and solve for length:

Length goes with the square of the beat time, so halving the tempo needs four times the length. Measure L from the pivot to the bob's centre of mass. A plain uniform bar behaves like a simple pendulum two thirds its length, so build it 1.5× longer. Keep the swing under about 15° each side, as wider swings run slightly slow.