If playing 6/8 to a metronome and checking bpm you must be aware that the metronome needs to be set to 6/8.
Measuring bpm with 6/8 can lead to confusion.
In 6/8 there are two main pulses, on beats 1 and 4.
1, 2, 3, 4, 5, 6
Each of these six beats is an eighth.
Two eighths = a quarter
Three eighths = a dotted quarter
A dot against a note / beat extends its duration by half again.
If the tempo is given as 109bpm, you need to know if this is the bpm for quarter notes. That would mean a tempo of 218bpm if considering eighths.
Divide that by three to get the tempo for the dotted quarter = just below 73 bpm. Try the metronome at 73 (for two beats per bar).
I hope that helps.
Cheers 
| Richard_close2u | JustinGuitar Moderator, Guide & Approved Teacher