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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