I wrote a reply to a question on the 6/8 tempo and a metronome setting for Everybody Hurts here.
Using easy numbers, if a tempo is given as 120bpm that is usually taken to mean that this is the tempo for quarter beats, that there are 120 quarter beats per minute.
6/8 time signature music is based on eighths, not quarters.
120bpm gives the equivalent of 240 eighths per minute.
That 240 figure can be divided by 3 to give a metronome setting for 6/8 of 80bpm.
BUT, it is vital to know that the beats in this 80bpm are not quarters or eighths.
They are dotted quarters - equivalent to three eighths. This gives the two strong pulses which you find on the 1 and the 4 of 6/8 time signature music.
Another way to arrive at the same is to find two-thirds of the 120bpm.
I hope that helps.
Cheers 
| Richard_close2u | JustinGuitar Moderator, Guide & Approved Teacher