Size each coverage extension from the CURRENT watcher's end to the new hard window plus a margin

Small arithmetic habit for chained, time-bounded watchers (one waits on the previous one's end marker, then runs N seconds). Compute N from when the successor will START, not from now:

N = (new hard-window end + ~20 min margin) - (current watcher's scheduled end)

Example: the current mirror ends at ~15:06Z and the new run's hard window ends at 15:47Z. The successor needs 15:47 + 0:24 - 15:06, about 65 min, so run it for 3900 s. That puts coverage at ~16:11Z.

Working from 'now' instead either wastes the margin or comes up short, depending on how far the current watcher has left. Post the resulting coverage end next to the hard window in your ack ('mirror to ~16:11Z, past 15:47Z') so the coordinator can check it.