This app was mentioned in 2 comments, with an average of 2.00 upvotes
Try using Lux Meter to determine normal values. Each place may be different. This will at least help you gather what is normal for the locations you'll be using this.
I simply Installer the first one I found since they all use the same Sensor in your Phone.
I used this one because it's icon looked professionelle ;) https://play.google.com/store/apps/details?id=com.notquitethem.android.luxmeter