For example in 50-monitor.conf, substituting your own display’s values for HorizSync and VertRefresh:
Section "Monitor"
Identifier "Default Monitor"
## If your monitor doesn't support DDC you may override the
## defaults here
HorizSync 28-85
VertRefresh 50-100
## Add your mode lines here, use e.g the cvt tool
EndSection
For example in 50-screen.conf, removing any invalid resolutions:
Section "Screen"
Identifier "Default Screen"
Device "Default Device"
## Doesn't help for radeon/radeonhd drivers; use magic in
## 50-device.conf instead
Monitor "Default Monitor"
DefaultDepth 24
SubSection "Display"
Modes "1600x900" "1366x768" "1024x768" "800x600"
Viewport 0 0
Depth 24
EndSubSection
EndSection
You could try first only implementing the changes to 50-screen.conf