The above questions are very much valid, although there is another point that you should be aware of.
If the sensors such as photoresistors normally operate at 5v and at half light they send 2.5v to your mcu. you would measure a 128 reading if the mcu's vref voltage was also generated at 5v and you were using 8bit analog .
The problem comes is if your batteries drop to produce 4v, your mcu is still running and maybe the comparator vref charges to 5v still, but the input from your photoresistors is 4v at half light so it would appear as 2v in and therefore register 102 which is a massive difference to the 108 that you were expecting.
The best way to do this is to see if the mcu has a vref pin and use it as the comparator voltage