I always play 16th notes on the metronome when I'm practising. I've got this little digital tuner/metronome and I suspect it screws up at places. I start around 60 and am reasonably comfortable on the acoustic at 110bpm.
I'm not great with time and I tend to screw up on the slower beats. I cannot play at less than 40 bpm to save my life. I should've gotten one a little earlier...like maybe 4 years ago
