Your calculations will save me quite some time calibrating. I'll change the resistor to 4.7k to boost the readings a bit, and using the AnalogReadSerial code for calibration is an excellent idea!
Too bad about the response time of my sensor though, that will probably complicate things a bit. Thank you for pointing that out, then I won't be as puzzled if my measurements go way above the expected values. I'll look into getting a photo-diode sensor with a better response time.
Thank you very much for your help! : )