Syncing two clips to one audio track

I am shooting an interview with two cameras; one camera is connected to a lapel mic, which is the main source of audio. To synchronize the second camera with the audio of the first camera I clap my hands loudly to cause a spike in the audio of both devices as a guideline to where I should "mark in" the clips; I know it is probably amateurish, but it works...usually. In this instance, when I try to "mark in" at the same points using FCP I am unable to set at the exact fraction of a second, resulting in one piece of footage 1/4th of a second ahead of the other.
Is there a way to adjust the audio to be editable at 1/60 of a second (current setting is 1/30 of a second) or a recommended method of synchronizing two video clips to one audio track?
Thanks!