Dear respected sodlei,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, to set typographically correct line spacing (1.2x text font size) in Word 365,
first you may need to select the text or entire document. and click the small arrow icon next to the Line Spacing button to open the "Paragraph" dialog box, and navigate to the "Indents and Spacing" tab. In the "Line spacing" dropdown menu, choose "Exactly" and enter the value "1.2" (for 1.2x line spacing).
In your case, you want 1.2 times the text font size. If your font size is, for example, 12 points, you'll set the line spacing to 14.4 points (12 points * 1.2).

We appreciate your understanding that sometimes the initial suggestions may not resolve the problem very soon. However, we can work together to narrow down and resolve the situation.
Appreciate your patience and understanding and thank you for your time and cooperation.
Sincerely,
Waqas Muhammad