Calculated average value isnt correct

Anonymous , that's interesting. Try adding some more debug code. I'm interested to see if the two methods you use to refer to [H2SO4 kg/d] give the same result.

    VAR H2SO4measure = [H2SO4 kg/d]
    VAR H2SO4column = CALCULATE ( SUMX ( ImportOfLIMS, 'ImportOfLIMS'[H2SO4 kg/d] ) )
    VAR Bleed = [APL Bleed m3/day]
    VAR H2SO4mg = [H2SO4 mg/l]

    RETURN
        FORMAT(MINX(MostRecentDates, [Date]), "YYYYMMDD") 
        & " ; " & FORMAT(MAXX(MostRecentDates, [Date]), "YYYYMMDD") 
        & " ; " & MINX(MostRecentDates, [@H2SO4]) 
        & " ; " & MAXX(MostRecentDates, [@H2SO4])
        & " ; " & FORMAT(Result, "0.00")
        & " ; " & FORMAT(H2SO4measure, "0.00")
        & " ; " & FORMAT(Bleed, "0.00")
        & " ; " & FORMAT(H2SO4mg, "0.00")

  If 'ImportOfLIMS'[H2SO4 kg/d] is actually a measure and not a column, then I believe you can change the [Avg_10_H2SO4_(kg/d)] to refer to it like this below. ADDCOLUMNS is an iterator and will calculate the expression for "@H2SO4" for every row in the AllDatesPrior table variable, so there is no need for a SUMX or CALCULATE.

    VAR AllDates_H2SO4 =
        ADDCOLUMNS (
            AllDatesPrior,
            //"@H2SO4", CALCULATE ( SUMX ( ImportOfLIMS, 'ImportOfLIMS'[H2SO4 kg/d] ) )
            "@H2SO4", [H2SO4 kg/d]
        )

 Incidentally, from the raw data, what actually is the correct result for 21 Jun?