Lengthy story here but in short, I followed some tutorials saying that to get an accurate size of your file in Photoshop when using the "print size" option, you divide your screen's current pixel size with the screen's physical horizontal size (in my case 1920px / 52,8 cm = 36.3636...). then you enter it under the menu "screen resolution" and in my case changed the unit to centimeters instead of inches.
After changed settings I start a new A4 sized document (international paper) and use "print size" on it. And when holding an A4 paper (210mm in length) against the screen, the Photoshop file's length is actually 207mm. Ok, so I tried to make minor adjustments with the decimals (from 36,36 up to 36,99) but nothing changes until the number becomes 37...then the Photoshop file suddenly decides to change size and jumps up to 213mm instead!! Why? The setting allows 3 digit after the decimal...why can't it use them for minor adjustments??