I am sure it has been covered before, but what is the optimum spacing for subsoiler legs v working depth? I know the shallower you work the closer the legs need to be but I seem to remember somebody (Philip Wright?) coming up with a formula.
It depends on your wing width as well I think, My spaldings flatlift has 5 legs spaced 750mm apart with 300mm wings, working to about 12in deep most the time for me, next year I am going to add a leg so spacing will be 600mm (overall working width reduced slightly), reduce wing width to about 200mm, depth will remain similar but perhaps a little shallower 9-11 in.
Doesn't really answer your question though!
Is 9-12" really subsoiling? Plough deeper than that!