Each buffer should be treated as if it were divided into three equally-sized regions. Conceptually, the first 25% is the green zone, the middle 50% is the yellow zone, and the last 25% is the red zone.
If penetration is into the green zone, or if there is negative incursion (tasks finishing early), no action is required.
If penetration enters the yellow zone, the problem should be assessed and various courses of action considered.
If penetration reaches the red zone, corrective action should be taken. Corrective action should involve ways to finish uncompleted tasks in the chain earlier or ways to accelerate future work in the chain to bring buffer penetration back out of the red zone.