hi, Anonymous 

try 

= iif (
     OR( [StationName.Value] = "CONGbann01",
         [StationName.Value] = "CONGccbr01"
     ),
     (Fields!DriftCorrection_pH7Value.Value + Fields!DriftCorrection_pH4Value.Value)/2.0,
     (Fields!DriftCorrection_pH7Value.Value +Fields!DriftCorrection_pH10Value.Value)/2.0
  )