Airburst height varies based on warhead size. Bigger the bomb, the higher the damage-maximizing airburst altitude. And don't be entirely confident about where

Airburst height varies based on warhead size. Bigger the bomb, the higher the damage-maximizing airburst altitude. And don't be entirely confident about where a bomb would be detonated. Nearby explosions can push a reentry vehicle off course. (RVs can't maneuver, so it can't adjust course when pushed. Not that there's a lot of time to steer at Mach 23.) Maximum accuracy requires GNSS satellites to be intact and the RV to be able to hear them, neither of which may be true in a full nuclear exchange.