Raymarine e7 User Reference page 287

refer to the documentation that accompanies the unit for
instructions on setting-up shared brightness.
8. Once all required displays have been added to the same
brightness group, select Sync Brightness on the multifunction
display.
A shared brightness message is displayed.
9. Ensure all displays in the brightness group are powered on.
10. Select Sync.
A message is displayed confirming that the brightness levels of
all displays in the group are now synchronized.
Customizing your display
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