Water treatment plant capacity
Well from my knowledge it is a bit more complicated than doing a calculation between potable water plant flow and wastewater plant.
If you already have datas, you can evaluate your average flows for each seasons and your peak flows for normal consumption.
Then you should evaluate any addition of industrial, residential and commercial that you plan to do for the futur before doing any design for the upgrade and not have to do another one in few years.
An other point that should be evaluated and added to the regular average and peak flow is the fire protection flow, pressure and required volume for fire protection. Check with your regulation if it should be done following the NFPA code or any other regulations (sometimes some cities, states, provinces or even insurance companies of your lmunicipality, have specific requirements that yu need to meet).
If you don't have any data, it should be evaluated with population and comercial and industrial use.
As you will see, there is an exhaustive study to do before planning an enlargement or a new plant.