Setting the DC control to the same number as the aperture is the maximum effect, as I understand it.
Setting the control to 0 results in no change, rather the lens performs as if the DC control was not used.
Lens aperture at 2 and DC control at 5.6 is a minimum effect, for example.
The DC control as neutral is the minimum effect, larger the number either direction from that the greater the effect is.
Generally the guideline is that if you set the DC to the same as your working aperture the effect will mostly be confined to the out of focus areas of the image making either the front or rear smoother and the opposite harsher. If you choose a setting beyond your working aperture(so for example shooting at F/2 but with DC set to rear F/5.6) then the effect becomes stronger also having a significant impact on the in focus areas of the image more akin to a typical soft focus lens.
This image was shot at F/2 and rear F/5.6 for example...
The other thing to consider as well is that when you mess around with the DC it also alters the focus, both in terms of shifting the focus point and in the cameras own focus reading. The latter is really the main weakness of the feature I would say as if your using AF(even just to confirm manual focus) you need to constantly change your micro focus adjustment. The easiest way to work is to just use a magnified view on the rear screen.