Ditto what MBJ said. Especially for those who might start without even thinking, literally the force of habit, and catch themselves in the middle.
Just to clarify the obvious, the purpose of the 90-day chart is not to give a din v'cheshbon or a moral assessment of one's tzidkus/madreigah/success rate, or even to give a definitive scientific confirmation that the person has acheived the magic psychological phenomenon 90 days clean. It is merely a motivational tool to get people started in their recovery, and an excuse to make a lechayim when they reach 90. And by its nature, it needs to have some sort of reasonable way of distinguishing between a slip and a fall.