Dear Denis,
> explain me the command line
This command line substitutes the value of the white level (the tag we discussed above) with the value of 53 thousand, 53000.
> why did you choose 5300?
Not 5300, but 53000. Please you open the file in RawDigger and switch off the black level subtraction (press the Black Level button on the lower left corner of the main screen and uncheck the Subtract Black). Next, display the histogram (Window - Histogram). You will see something like

Switch on "Linear X-Axis"

The hump in extreme highlights is non-linear / noisy zone (essentially, the hump for the blown-out zone looking like Gaussian distribution instead of the single value indicates it contains just noise due to pixel non-uniformity), and needs to be trimmed off. You can see it starts somewhere around 53000. We can study this hump in more detail, switching off "Auto" under "Linear X_Axis", setting the "Range:" start (left field controls the start, right field controls the end of the range) to 50000, and decreasing the "Bin Size" (set it to 1, and it will be re-calculated to the minimal possible automatically; 8 in my case):

53000 seems to be a reasonable estimation for the beginning of the hump.
> I tried the same value with other images and I didn't get the same result (sometimes grain disappears but with a bigger overexposed area).
The difference between 60074 and 53000 is less than 0.2 EV (calculation is log2(60074/53000) = 0.18 EV) However, the real maximum value depends on the temperature of the sensor and surrounding electronics, and on sensor fatigue. So you may need either to examine the last dng for each shot to establish the maximum for the shot (the last frame usually exhibits the effect in its strongest), or stay on a safe side, which may be 1/3 EV from the 60074, that is 47700.
> I also tried to process a group of images (the same Magenta_test.dng copied 3 times)
I made a dir on my desktop, with the name "1" and put there 5 copies of the file, adding suffices _01, _02, _03, _04, and _05 to the filenames. Next, I applied the following command, telling exiftool to process all dng files ("-ext dng" stands for all files having dng extension) in that directory (the last parameter "/Users/iliah/Desktop/1/" is just the path to the files, without filenames):
exiftool -IFD0:WhiteLevel=53000 -ext dng /Users/iliah/Desktop/1/
It updated all 5 files. To check, I issued the following command:
exiftool -WhiteLevel -ext dng /Users/iliah/Desktop/1/
The result was that all the files received an updated white level:
======== /Users/iliah/Desktop/1/Magenta_test_01.dng
White Level : 53000
======== /Users/iliah/Desktop/1/Magenta_test_02.dng
White Level : 53000
======== /Users/iliah/Desktop/1/Magenta_test_03.dng
White Level : 53000
======== /Users/iliah/Desktop/1/Magenta_test_04.dng
White Level : 53000
======== /Users/iliah/Desktop/1/Magenta_test_05.dng
White Level : 53000
1 directories scanned
5 image files read