NOTE: One who came late may NOT start S”E unless he knows that he will be able to reach Elokai Netzor by the time that the chazzan reaches Kedusha. The only exception to this is where he will miss zman tefilla.
See Mishnah Berurah 109:2 and Beiur Halachah, s.v. ha-nichnas.
Mishnah Berurah 109:14; Aruch ha-Shulchan 109:11-12.