the (max error) should be 2-3 K in the permitted range of application (see the standard for additional information).
When calculating water dew point make sure that liquid phase is almost all water, in your example if you reduce the water amount to 50 ppm (C1 0.8 C2 0.1 C3 0.049995 CO2 0.05 H2O 5.0E-5 ) then at 20 Bar.a , dew point 231 K , the condensate includes water and all other components (i.e. the gas can contain that amount of water)