Automatically redistribute the target per customer

I've created a simplified model based on your description with the following measures that you should be able to modify to get the result you're looking for.

Sales Measure:

$ Sales = SUM(Sales[SaleAmount])

Portfolio Target Measure:

$ Portfolio Target = 
CALCULATE(
    SUM(Goals[Target]),
    FILTER(
        Customers,
        COUNTROWS(VALUES(Customers[CustomerCode])) > 0
    )
)

Customer Target Measure:

$ Customer Target = 
VAR _CustomerSales = Sales[$ Sales]

VAR _TotalSales = 
CALCULATE(
    Sales[$ Sales],
    ALL(Customers),
    VALUES(Portfolio[PortfolioID])
)

VAR _PercentToTotal = 
DIVIDE(_CustomerSales, _TotalSales)

VAR _PortfolioTarget = Goals[$ Portfolio Target]

VAR _CustomerTarget = 
_PercentToTotal * _PortfolioTarget

RETURN
_CustomerTarget

Result:

Consider providing a dataset to work with on future requests; this will make it much easier to answer your questions.

If this solves your problem, please mark this answer as the solution.