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The 20H rule has nothing to do with crosstalk. It is supposed to be a method of reducing emission from the edge of a circuit board where short wavelength RF is present. The idea is to pull back the edge of a power plane by 20x the height above the adjacent ground plane to change the field shape at the edge of the board.
There is a lot of controversy about the validity of the theory. You can find several papers on the subject, but one recent one is:
The 20-H rule is a printed circuit board layout guideline.
On boards with power and ground planes, the fringing field at the edges of the board is contained by backing the edge of the power plane away from the edge of the board by a distance equal to 20 times the separation distance between the planes.
Studies show the radiation from a board implementing the 20-H rule is actually slightly higher than the radiation from boards with a traditional design.