Photoshop's "print size" is still not accurate despite changed settings (Photoshop won't use decimals?)

Question

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??

Billy Kerr · Answer

To get the Print Size functionality to show the physical size of a print document on screen, follow these steps:

Let w be the width of your screen in cm (obviously you need to measure this with a ruler).
Let x be the width of your screen in pixels (find this in your display settings)
Screen resolution = x / w.  Round the value to the nearest two decimal places. This should be accurate enough. 
Create a new print document using the print presets in Photoshop, something like an A4 sheet will do. Make sure you set the units to centimetres, and the resolution to 118.11 pixels per centimetre (equivalent to 300ppi).
Go into Preferences > Units and Rulers, set the screen resolution you calculated in step 3, in pixels per centimetre. The print resolution setting should be 118.11 pixels per centimetre.

Now try View > Print Size. Then, with a ruler, measure the width of the sheet on screen. It should be 21cm.

Note: for consistency and to avoid confusion I made all measurements metric, however you can also use imperial measurements if you want. It makes no difference.

Photoshop's "print size" is still not accurate despite changed settings (Photoshop won't use decimals?)

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