I think there is another factor that needs to be considered in the discussion and that is the type of in-camera metering pattern. A spot meter and an averaging meter that meters a much larger area will each be affected differently. As with most photographic calculations, the ranges suggested are only a beginning point and the owner will need to experiement with each camera to see what factor is needed for each camera/meter used.

I have a circular 62mm Nikon for my Bronica that is an averaging meter in the AEII and the suggested compensation factor is pretty close but using the same filter, by holding it in front of the lens, on my Leica CL with a spot meter the factor is more variable de[ending on where and how I use the spot meter. If I meter a grey card it comes close to the Bronica but if metering a scene, it can be as far of as an 1/2 stop.

The circular polarizer was developed because of the AF and not the in-camera light meters just to add some clarity.