Hmm useless for people like me who have hardware calibrated screens - makes a 120cd/m2 screen look well odd
120cd/m2 is the perfect luminance for comfortable eyes so if you can you're better off borrowing a colorometer and calibrating all your screens.
You'll find almost all screens today default to well beyond 250cd/m2 which is just way too bright and while not a health risk, it will cause fatigue and eyestrain in the dark.
SOunds like a daft app. Just use background/ambient lighting surely or turn the contrast down on your screen. A lot of them automatically adjust if you want them to now.