Impedance Phase measure ADuCM350


I am working on "ImpedanceMeasurement_4WireBioIsolated" example for ADuCM350. I have read different application notes and I know that phase measure is not accurate for this system, but I have read one thing that i didn't understand. There is an application note called "Bio-Impedance Circuit Design for Body Worn Systems" written by Jose Carlos Conchell where at pag 9 states that the phase of the unknown Z should be computed as 180+(Ph(V)-Ph(I)), is this formula right? Do I  need to add a 180 degrees shift to the phase measured? Why?

Thank you