I am not sure that I am the best guy to advise, but let me just waffle for a minute.
We are trying to calculate the voltage impressed on the blue line by the voltage on the red line.
If the voltage on the red line was a sine wave, and, if Rvictim was "very high", then the simple capacitive divider formed by Cadj and Cgnd-v could be used as in the top expression in the attachment.
However, if Rvictim is sufficiently small to be of a similar order to the reactance of Cgnd-v, the the simple capacitive divider model will not work.
But, the redline voltage is not a sine wave but a step, so we are trying to allow for this by looking at the rise-time of the step on the red line and the time constant of the blue line, and applying a correction factor 1/(1+k) to the simple voltage divider.
This is where my knowledge breaks down, I have never seen (or at least can never remember it) this correction for risetime applied like this.
At a first look it seems to make sense.
An agressor waveform with a rapid rise time (Taggressor = small) will have a less effect on a victim with a slow rise time (Tvictim = large) hence K is small, than it will on a victim with a fast rise time (Tvictim = small) hence k is large.
At this point it is lunch time and my poor little brain is feeling confused.
Does this diatribe help you at all?
JimB