50% will give smallest all-round vibes, but may not be the best, because
who cares how small the vibes are in the direction you can't feel?
Most factories went for more like 65% if vertical cylinders like yours,
so that the up/down vibes you can feel are less, forward/reverse vibes
that aren't so obvious are more.
Resonant frequency of frames will be different in each direction, so BF
is sometimes adjusted to steer the vibes away from problematic resonance.
Note that Norton 360 degree twins were originally around 65%, then when
they started to rubber-mount the engines they went to 50%.
There were exceptions, some Velocettes reportedly 85%, so good luck!

Jordan

wrote:
thanks for all the info, guys. so the answer seems to be 50% to 70%;
quite a spread.