RAW, changing your default light level is a zero point feature. Realistically, it should probably be a net disadvantage if it applies to the entire zero penalty range; if you want to be a bit fairer, I would use my original light level table
here and assume that you can shift your zero penalty range (levels +0 to +4) as a zero point feature. Thus, a purely nocturnal species might be fine at light levels -1 to -5, and would be at -5 in full sunlight.