How should I install a macrometer at the entrance of a DMC?
Macrometer is not a standard US water term. The definition on Wikipedia is "A macrometer is an instrument for measuring the size and distance of distant objects." I don't think that is what you mean.
Are you talking about a master flow meter measuring total flow into the system?
If you are using it for calibration, you would make it a SCADA signal element.