The concept of the Preinking curve is that you can can be very close to the desired ink density faster.
Drag the point right off the grid.
There are two other tools to help you work in Curves.
Presets Just by way of mention, you should try the Curves presets in the menu to get a feel for what particular points can do to a curve.
Set at the maximum of 2880 dpi.
Specifically, the selection lasso stroke and the bounding rectangle that highlights the selected strokes.