First of all  you can't just ...

First of all  you can't just dose chlorine or chlorine by products without knowing the physical and chemical  characteristics of the water you're about to disinfect, for instance the dissolved iron content and turbidity of the water. Dissolved iron because iron react with chlorine or chlorine by products and until and unless you what the iron content is or the free available chlorine after reaction you might not be able to attain efficient disinfection. Secondly, the turbidity of the water also plays a role in efficiently disinfecting the water because it shields the microbes from the lethal effect of the chlorine. So, after establishing the turbidity and to a large extent the dissolved iron content of the water, get three or five dark bottles that can contain at least 1000 ml of water.

Dose a different quantity of the disinfectant into each bottle containing the 1000ml of the water to be disinfected, then allow it contact time and measure the free and the residual chlorine. Remember beside chlorine being an efficient disinfectant it is also a carcinogenic substance and it can also create taste in the water. So, to be fair to the people you're assisting this  series of test called " Break point Chlorination"  could be carried out for a whole year in order to establish the seasonal variation in the water quality and with that the people will not concern themselves with all the calculations you're making reference to in your question.