Tank water level fluctuation based on D-1 consumption
Why working with1? It can be risky if you have a fire or a pipe that would break. Why not doing a PID on the water tank level taking into account the lowest level (fire protection and maybe chlorine contact time) and the departure of a second pump if the level continu to lower when your first pump is a 100% speed. You may know your normal demand but you never know when you can have an increase for some particular reason. So I would say that control on actual level of reservoir would be much safer.