Cultivators facing their heavenly tribulation usually see the tribulation clouds descend from ten thousand meters high in the sky. Until it gets within 3000 meters of the ground, it's often too far away to be observed without technical means.
The extreme attack range of a cultivator is around 3000 meters. Beyond that, it's not that one can't attack, but rather that divine sense struggles to lock on. Yan Yu timed the activation of the Yin-Yang Dust Sword Formation just as the main body of the first wave of tribulation thunder dropped to a height of 3000 meters.
Below that, from 1800 meters to 500 meters, is the best time to counter the tribulation thunder. On one hand, the tribulation thunder is still at a distance from the cultivator, and on the other, it's the time to lock onto the target from afar with the Flying Sword Magic Treasure, reducing the number of tribulation thunders.
