When is mark-room actually decided — at the zone or at the mark?
At the zone. Rights are a photograph taken the moment the first boat reaches the zone, three hull lengths out — nothing you do afterwards changes it. A boat clear astern at that moment must give the boat ahead mark-room under Rule 18.2(a)(2) and may not sail into the gap between her and the mark. A boat overlapped outside at the zone owes mark-room to every boat inside her under Rule 18.2(a)(1) — one zone, two snapshots, two different obligations.
<iframe width="560" height="315" src="https://www.youtube.com/embed/h-s6L4pC6vo" title="Sailing Kiwi" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://sailing.kiwi.nz/v/h-s6L4pC6vo/">Source: Sailing Kiwi</a></p>