MetricAchieved is already a SUM measure. Drag MetricAchieved into the viz and tableau will aggregate from there if necessary.
My thoughts were the same.
1. create a boolean calc field that tells me if they've hit the target or not.
2. sum those occurrences.
you could do:
1. Calc Field: ACTUAL = performed/denominator
2. Calc Field: Hit Target = IF ACTUAL>=Target THEN 1 ELSE 0 END
3. Calc Field: Sum of Hits = SUM(Hit Target)