Tap on the beat for at least eight taps — the readout uses the median of your intervals, so one late tap won't drag the number. Keep going to ~16 taps and the confidence figure settles. If a song feels ambiguous (is it 80 or 160?), both are "correct": that's the half-time feel, and DJs argue about it professionally. The "nearest common" figure snaps to the tempos most music actually uses.
Want the click to play back at the tempo you found? Take the number to the metronome. Tuning the instrument that plays over it happens at the pitch detector.