I think the gist of it is that the "threshold" knob label actually describes the knee point... while the deltas for the compression curve exists both below and above that mid point.
In other words the onset of compression begins at the delta (or point of curvature) below the knee point, (or point of intersection) described on the threshold knobs readout or scale.
Some one pointed out that in a system such as this that a softer knee would have the delta, or point of curvature, or the onset of compression further below the knee point.
The point being that adjustments to the knee character will also effect the level at which the onset of compression occurs.
The softer the knee the earlier the compression kicks in.
Think about that until it makes sense to you... it's a key to understanding what is going on with 4k controls.
So, with PC4k compressor you are totally dependent on viewing the gain reduction meter and guessing where the onset of compression is while using your ears to decide if you like what is happening.
Personally I prefer a modern, up to date, fully featured compressor that allows me to have everything you get with Pro Channel and so much more.
best regards,
mike