How, specifically, is render convergence calculated?

I know the general idea of WHAT convergence is but I haven't been able to find specifically HOW Daz Studio calculates that percentage. The closest I've found is a forum post from 2020 stating:

"Convergence percent = Illuminated pixel count by rays / white noise pixels (pixels yet to be hit by rays)."

But that doesn't seem to be the whole story.

I'm rendering an enormous image (7500x10000) which has been chugging along for almost 36hrs and still measures 0% in the render progress window. I can CLEARLY see improvement in image quality as the iteration count goes up and I've noticed that the convergence percentage regularly stays at 0% then suddenly jumps to 40-60% on most renders. The sheer size of this one seems to have delayed that jump. I have to believe that the raw pixel count makes a difference but does anyone have access to the actual formula? Quantity and complexity of light also seems to factor in.

I'll usually set the convergence high enough that I'm sure it will look good if it finishes in the background. I regularly cancel early if the render looks good enough to my eyes and I'm almost to that point with this big render. I'm aware that this knowledge isn't really necessary to use the software but I like knowing how things work and it got me thinking as to how, specifically, Daz Studio calculates render convergence percentage.