I'm using the same keyboard and I had the same problems since version 3.
In version 4 I used the keyboard calibration assistant and the KS88 built in velocity curve #5 and it ended up with a velocity curve (in pianoteq) like this:

X axis     Y axis
------------------
0             0
95           40
112         64
121         96
127         127

(Sorry I couldn't post the image).

You can try this as a starting point. Personally, I am quite happy with this setting, but take care that piano is not my main instrument so you may have a different taste and you will have to make some adjustments.

Start say, by using the Moderately Fast curve. Since you're reaching 127 too easily (and never want to hear it again?), pull down the rightmost point some (until as hard as you can hit say Middle C and nearby is acceptable). Then you've got your basis for pulling down the points leftward of that output level (none should be higher than the rightmost, let's say).

Working in from the other end, if Moderately Fast's influence quietens as soft as you want to play overmuch, pull up the lowest inner point (your very leftmost point should be zero volume) until it's passably soft for you, then you have the basis for making points rightward of that as high or higher.

And of course make the leftward working-in-adjustments (louds) meet the rightward working-in-adjustments (softs) smoothly in the middle. See how that all results, for you.

I believe Pianoteq interpolates between points curvewise, not straightwise. And a click between existing points will insert a new point between, which can be dragged around until perfect and removed with a double-click.

But don't believe me till you've seen it work!

Try to adjust the graphic setting the extreme right side of the graphic bellow the 127 value.

Something like this (image on the right, ignore the left image):

The middle anchor point you can ignore,,or positioned upper or lower, the one gets better.