Well, is not so straight forward … You go to X Server Display Configuration and make sure the values are the correct ones
Then lower that window you have some buttons. You click on Save to X Configuration File
But you are not doing so, because you click on Preview in the popup window
Make that popup window larger so you can read, and you copy the entire Section "Monitor"
Then you open up the /etc/X11/mhwd.d/nvidia.conf with Kate and paste that in place of the current Section "Monitor"
Hope this helps.