Extensions answer a different question from retracements: if the impulse resumes, where might measured moves land? The 127.2% and 161.8% projections are popular because they often sit near prior structure — not because they are magic.
Project from the same swing you used for the retracement. Mixing anchors is a common source of optimistic targets that float in empty space. Once the extension is drawn, look left for supply or demand that already reacted. If nothing sits nearby, treat the level as a soft waypoint, not a destination you must hold for.
Your trade plan should say what happens if price stalls early. A partial at a nearer structural high, with the remainder conditional on acceptance, beats a single distant target that turns a good idea into a stubborn one.
Invalidation belongs on the page before the extension does. Without it, the projection is decoration.