I got to poking around this issue today. Sadly, the graphs that used to be available at some M3 site seem to be NLA because the site is gone. I did find a reference tho to rear camber gain being 0.9deg per inch of compression. The problem with that # is that it came from a guy with sport springs and we have more drop then he does. This is an issue the camber gain is almost sure to be non-linear, therefore his -0.9deg per inch of compression might not be accurate for us. My recollection of those graphs is that they also showed that camber gain was nonlinear.
With my car on the lift I raised the car up in ~10mm increments and measure camber each time. I also put a 100lbs of weight on a rear corner and measured the camber with the 4mm of compression that resulted.
Bottom line. The -0.9deg of camber gain in the first inch of compression is a reasonable ballpark. The second inch of compression will cause more neg camber gain. I’ll swag and call it -1.0deg gain in the 2nd inch of compression.
I looked at a rear spring for a moment and guessed that it would be willing to compress about 2.5" before the coils bound up. Call it ~2.6deg of camber gain at full compression.