Don't think of it in terms of "overlapped with" the signal boat. Instead, concentrate on "room to keep clear" of the signal boat. When L changes course to shut W out, she needs to give W enough space that W can avoid the mark (and meet her other obligations) by maneuvering promptly in a seamanlike way. Certainly, if W already has her bow under the stern of the signal boat, L has waited too long. It may, however, also be too late even before W reaches the signal boat. There's no set distance or time; L simply must give W enough space to peel away in the existing conditions.