When a pair's gap differs from the scheduled gap by more than:

Set either half to 0 to judge on the other alone — on both lines, since one line testing what the other ignores is not an earlier warning. “and” is the default: ±25% alone fires at one minute on a four-minute headway, and ±5 min alone fires on a forty-minute one. A watch set at or past the alarm is pulled back inside it (to 60% of it) — it is the earlier line, and a watch band nothing can reach is no warning at all.