Q. If you are davening with a minyan, but are behind the chazan, (you came late or the chazan is too fast), when would you say or not say “Kel Melech Neeman” in Shema?
If you start Shema before he ends? You are saying birchas Kriyas Shema and hear him finish, then you start Shema? Still before Borchu? Is there a cut off?

A. Remoh (O.H. 61: 3) mentions that they are 245 words in the three parshios of shema, and 248 members in the human body. In order to complete the words to match 248, the shaliach tzibur repeats “Hashem E’ Emes” at the end. Remoh adds that when one prays without a minyan and won’t hear these three words, he may add Kel Melech Ne’eman at the beginning.
Horav Shlomo Miller’s Shlit”a opinion is that from the above it stands to reason that the three words added by the prayer leader, will have effect only when the late coming reader is already engaged in reading the shema and listens to them. Otherwise he may add Kel Melech Ne’eman at the beginning of his reading.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a