There might be another way to go, as well.
Instead of tightening the focus on the nostrils in order to set aside the other distracting input, you might try blending all the input together such that, no matter what else can be detected about the body, the breathing is noted as a repetitive body-flexion encompassing all the other bodily inputs. This, then, can be the basis for the third step of anapanasati's first tetrad.
"And how is it, bhikkhus, that by protecting oneself one protects others? By the pursuit, development, and cultivation of the four establishments of mindfulness. It is in such a way that by protecting oneself one protects others.- Sedaka Sutta [SN 47.19]
"And how is it, bhikkhus, that by protecting others one protects oneself? By patience, harmlessness, goodwill, and sympathy. It is in such a way that by protecting others one protects oneself.