Very good question. Unfortunately, I am going t sound like a lawyer when answering your question, as the answer is 'it depends'.
Yes, you definately will need to over build the amount of UV treatment. Maximize the time and intensity for UV exposure.
The reason why this will need to be overbuilt is that there will always be upsets in the plant (unintentional, and intentional). These upsets will always cause 'caking' and dirtying of the UV tube(s), which will decrease the effectiveness of the treatment.
NTU's is the standardized measurement, and is a pretty good method for translucence. I would try and use this type of data for quality control.
As far as information is concerned, keep looking up more information from WHO. There are many books on Water and Wastewater treatments that will be able to help you with design parameters.