We are not disagreeing here, I think your 60 min calculation is correct. My point is that your calculations are at 50 cm and you get a much lower time for expos

We are not disagreeing here, I think your 60 min calculation is correct. My point is that your calculations are at 50 cm and you get a much lower time for exposure if you bring your light bulb much closer and figure out a way to reflect the UV light so that more light from the bulb hits the intended target.