The calculation of missing transverse momentum (ETmiss) has been and will continue to be vital for physics analyses at the ATLAS experiment. Quantifying the momentum imbalance of the event gives physicists the ability to identify non-interacting standard model particles, such as neutrinos, and to search for signatures indicating the presence of physics beyond the Standard Model. In order to determine if the observed ETmiss comes from real non-interacting particle(s) or rather purely detector effects, such as jet mis-measurements, a ETmiss significance was defined. This significance improves the separation of fake sources of ETmiss from real ones. This poster demonstrates the quality of the modelling of the ETmiss significance using the full Run 2 dataset. The analysis is performed using a standard candle with low real missing transverse momentum: Z boson production with decays to two opposite signed leptons. Two different reconstruction schemes for jets, calorimeter only and a new particle-flow method, are studied. The agreement between the data and expected Standard Model processes is shown to be very good.