OR function

Anonymous Well, here is the code cleaned up:

Yield %_latest_total =
IF (
    ( Fact_Yield_Detail_Latest[Item_Group_deduce] = "AP3" )
        || ( Fact_Yield_Detail_Latest[Item_Group_deduce] = "AF1" ),
    CALCULATE (
        SUM ( Fact_Yield_Detail_Latest[Good_Qty] ),
        Fact_Yield_Detail_Latest[Process_Description] = "Double Check"
    )
        / CALCULATE (
            SUM ( Fact_Yield_Detail_Latest[Start_Qty] ),
            Fact_Yield_Detail_Latest[Process_Description] = "Lathe Cutting"
        ),
    IF (
        Fact_Yield_Detail_Latest[Item_Group_deduce] = "SP2 (toric)"
            || ( Fact_Yield_Detail_Latest[Item_Group_deduce] = "SP2 (non-toric)" ),
        CALCULATE (
            SUM ( Fact_Yield_Detail_Latest[Good_Qty] ),
            Fact_Yield_Detail_Latest[Process_Description] = "Double Check"
        )
            / CALCULATE (
                SUM ( Fact_Yield_Detail_Latest[Start_Qty] ),
                Fact_Yield_Detail_Latest[Process_Description] = "UV mould prep"
            ),
        0
    )
)

What I would suggest is to return hard coded values for each possible condition. Then you can check your logic.