It took Kaido to fall from that height and to fall at that velocity, taking also into account his body mass and structure, to make him feel hurt a little bit. Following from that, we can logically say that luffy needs to punch (falling to the ground is almost the same as punching as both are more or less blunt force) 100x stronger than the force that Kaido had generated from hitting the ground, in order for him (luffy) probably to deliver a real-time scoring punch.

Estimation: Luffy needs to make one punch strong enough to destroy an entire island and thus consecutively deliver the same punch, in order to defeat that Kaido.