Just had another look at your post and I think the answer should be quite straightforward.
The Workflow Rule needs to run on Milestone items (All Item Types > Work Item > Milestone).
Set Run Time:
When a record is created, or when a record is edited and did not previously meet the rule criteria
Evaluation Criteria:
Run only when the following conditions are met
*If True = *
$Name = '2 - Tag Deployment' && $PercentCompleted = 100
Let me know if this works for you.
Jonathan, , Clarizen Customer Success