Has anyone come up with a torque spec for the straps? Or any way to know what is tight enough?
Just lean on the rear end of each tank and try to move it up and down. They should be quite firm and if there's excessive movement then the straps are too loose. I usually aim for movement of 5mm or less but it's not an exact science.
If you've run out of adjustment on the tensioner thread, try slipping a spacer between the strap and chassis where the strap feeds around the chassis tube just behind the seat back.
If the straps have been running loose, they are likely to be bunched up at the bottom of the adjuster bracket slot so don't forget to reposition them before re-tightenning.