Calculation using measure from one table/matrix and subtotal from another.

Since you have a M:M relationship between the Created By columns in your two tables, you could make a bridge table of unique CreatedBy values to make 1:M relationships to both tables.  Or, with no relationship between the tables you can use this measure in a table visual with the Created By column from your TaskDetails table.

NewMeasure =
VAR vREJ = [Unique REJ]
VAR vTotalObjects =
    CALCULATE (
        COUNT ( CFECODetails[Obj Type] ),
        TREATAS (
            VALUES ( CFTaskDetails[Created by] ),
            CFECODetails[Created by]
        )
    )
RETURN
    DIVIDE (
        vREJ,
        vTotalObjects
    )

If you are going to do a lot more with this model, you should consider making the bridge table to make your future analyses/visualizations easier.  There are lots of articles/videos on making bridge tables out there.

Pat