Most put tube about half way to about even divide tubes.
I put one 8.5" up from front iso mount tab, going by
universal structure wisdom that applies to everything
else but an isolastic Commando.
Stick a screw driver though naked mount tab and lever
on it a bit - let that torsion sense sink in for whats
happening on leaned powered road loads applied
by lever from rear patch to front mount >>>
I'd now put it closer to the mount tabs with more
tube twist resistance in mind rather than tube length
Mild steel thin 1/2" OD tube is what I had weld shop
do up. Also added one down at rear frame bend bottom.