I wish there was a legit way to tune the Auto mode to a user preference threshold (ie: +/-) via the device GUI. So instead I found this...
Under the Garmin\Debugging directory there is a "Backlight_Lookup_Table.csv" file containing these default settings:
---
Ambient Light Min (lux),Ambient Light Max (lux),Backlight Level (%)
0,1,30
1,300,60
300,1500,80
1500,65535,0
---
Adjusting the first 3 % values upward to say 55, 75, and 85 should provide a significantly brighter experience in lower light environments (ie: < 300 lux).
My brighter level of darkness ;) ...
---
Ambient Light Min (lux),Ambient Light Max (lux),Backlight Level (%)
0,1,55
1,300,75
300,1500,85
1500,65535,0
---
It should be noted (but as usual it seems not) that when Auto backlighting is enabled, the backlight timeout is disabled (ie: backlight is always on).
* CAVEAT: Garmin updates may overwrite this file. Keep a backup copy of your custom settings.
