Completed Work by cascade (Issue -> Epic -> Objective)

Hi ValerienSegard  - 

First thing to do is see if you can change your data model (inside PBI) to have Objective, Epic and Issue in 3 separate tables with 1-to-many relationships (1 Objective to Many Epic, 1 Epic to Many Issue). If you can do that, it will make things much easier.

Once you do that, create a column on Issue for the IssueDuration

IssueDuration = DATEDIFF(Issues[Start Date], Issues[End Date], DAY)

Then a measure for the rolled up Completion %

Completion Pct =
DIVIDE (
    SUMX ( Issues, Issues[IssueDuration] * ( Issues[Completion] / 100 ) ),
    SUM ( Issues[IssueDuration] ),
    0
)

Hope this helps

David