I dont know if you really can 'calculate' PUR. You'd have to measure it, I mean its a property of the light and depends on its spectral distribution and intensity across a given range. To 'calculate' it youd need some accurate data if not measured by yourself, from someone else or the company, and either a whole lot of math by hand or a program to interpolate the data.

As I understand it, LUX is weighted towards human sight, PAR is unweighted and accounts for all wavelengths 400-700nm, and PUR is weighted towards plants photosynthetic regions. The idea of PUR is good, but i've not seen any bulbs really measured with it and I have yet to hear of such a meter - is there such a thing experts?