My router seems to have gone down so you can't reach my blog or my PUR-calculator.

I calculate PUR exactly as outlined in this article:
Aqua Botanic-light bulb comparison
"Steps to calculate tabulated quantities"

It uses a photosynthetic action spectrum, which is an average of how most plants use the radiation. Crazy Loaches is right that it thus can't be calculated *exactly* for one single plant - but it will be very near.