Calculating stock based on weekly changes

See if this works for you. (BTW, there was an inactive relationhip in the model you sent, which needs to made active)

Since it's not clear whether you wish to see the value as the initial or the final stock of the week...

Initial weekly Stock =
VAR _Current =
    CALCULATE (
        LASTNONBLANKVALUE ( 'WeekYear Start'[Year-Week], [Sum Stock] ),
        ALL ( 'WeekYear Start' )
    )
VAR _RunningStock =
    CALCULATE (
        [Sum Weekly Changes],
        FILTER (
            ALL ( 'WeekYear Start' ),
            'WeekYear Start'[Index] >= MAX ( 'WeekYear Start'[Index] )
        )
    )
RETURN
    _Current - _RunningStock

Or...

Final weekly Stock = 
VAR _Current = CALCULATE(LASTNONBLANKVALUE('WeekYear Start'[Year-Week], [Sum Stock]), ALL('WeekYear Start'))
VAR _RunningStock = CALCULATE([Sum Weekly Changes], FILTER(ALL('WeekYear Start'), 'WeekYear Start'[Index] >= MAX('WeekYear Start'[Index])))
VAR _PW = _RunningStock - [Sum Weekly Changes]
RETURN
_Current - _PW

Or both even

Btw, I the table for the current stock will only ever have one value per product, you can probably use a simple CALCULATE([Sum Stock], ALL(Weekyear Start)) instead of the CALCULATE(LASTNONBLANK... in the _Current variables

I've attached the sample PBIX file