[kazinator]

I should mention that I know about Grain Extract's subtraction operation. I've used it for "leveling" also.

Subtraction would be fine if the data were logarithmic: that is to say, if the intensity information we are operating on represents decibels. We figure out the low frequency signal's "floor" over each area of the image, and simply subtract those decibels.

Division is better on the assumption that the intensity values of the pixels are linear. Division of linear samples is subtraction, in the decibel scale.

The right way is somewhere in between, because in RGB displays, pixels are put through some gamma curve. They are neither linear nor logarithmic.