Question on how to edit text margins (avoiding corner)

Hi everybody,
I'm a long time user of Calibre but just for pretty basic stuff.

My Kobo Mini is in its last days and nowadays it works partially with a (probably broken) "frozen" corner, but the rest of the screen still looks readable.

I found around the forum some old posts mentioning that some feature in the Calibre Editor allow to set a Top or side margin (with pixels? %?) so the text of the resulting epub file would fit into the working section of the screen.

Could someone give me a hint on how to do that (if it's possible)? I went around every option on the Edit section and I didn't find anything...

Thanks in advance!