OK, so this is solved for the next build.
To configure a two way binding with a curve like this, you should configure one binding with the normal curve, and the other with the same curve amount but the new “Inverse” version of the curve.
eg: One binding like this:
and the other like this: (note the curve kind)
Also note both bindings have the same “curve amount” and that you can now type this value (“0.55” in this case)
With earlier builds you can get something very close by using a negative curve amount, but since the curve isn’t perfectly symmetrical (though the line y=1-x) it’ll still jitter a little at one end of the range.
Finally, songs/racks saved in the new build that use the new inverse curve kinds won’t load in the earlier builds of Cantabile.
New build should be up soon.