The parallax map is a grayscale procedural map. Heights are calculated relative to the midpoint red127green127blue127, which is the relative zero of the surface, where 0,0,0 is the absolute depth, and 255,255,255 is the absolute height (in fact, the zero value is discarded by the engine, like white, must be at least 1,1,1 or 254 , 254,254).
To display heights correctly and avoid visual distortions such as spiky surfaces, stretching, and holographic fluctuations in the main diffuse map image, your height map must be plotted (read drawn in gray) within a very narrow range of values. It doesn’t matter where this range will be located - closer to black or white, adjacent areas should differ very slightly. To do this, on the finished map you need to use the Brightness/Contrast tools of your graphic tool (I use GIMP) to reduce the contrast of the image until you get the desired result.
The size of procedural maps does not affect the quality of the image in the game as much as, for example, a diffuse map, therefore, with a diffuse map size of 4096x4096, it is not at all necessary to have a normal map of the same size; from the point of view of the game mechanism, this is a waste of video memory of your video processor. I have experimentally found that reducing the size of procedural maps by half does not affect the image quality in the game. Therefore, you can safely scale them by 50 percent of the diffuse card, thereby freeing up your video memory for something else. Even more important is that heightmaps can generally reduce memory usage, since they only take gray values, so they don't need a huge size or an alpha channel. There is also a little trick in the final compression. The BC7 format is of course very advanced and all that, but! For a height map, there is a more suitable format - BC4u R 4 bpp gray scale. In the Nvidia texture tool, specify this format, specify the texture type as a grayscale and you will get maps of simply microscopic size that work perfectly in the game. So, for example, for a 2K texture the layout will be as follows:

Diffuse map (BC7) 5462 kb
Normal map (BC7 scale 50%) 1366 kb
Height map (aka parallax map) (BC4u grayscale scale 50%) 683 kb

As you can see, using three cards takes up less memory than simply using two cards at 2K scale without much benefit in quality. I will say more, with this approach, parallax works much better, less distortion, fewer artifacts, more performance. When used comprehensively in Whiterun, I got an extra 10 frames and less stuttering. Perhaps this information will be useful to someone.

A short video of how this mod was created (a tiny episode). I made the video for fun, but at the end I showed what settings to use.Turn on subtitles to understand what is happening on the screen.