Inventory Forecast Weekly

Hey, 

I had a similar situation to yours and solved it like this: maybe this will work for you too?

first i made a Weekly Sales Forecast. 

Weekly_Sales_Forecast = 
VAR MonthlyForecast = SUM('Sales Forecast'[MontlySales])
VAR DaysInMonth = DAY(EOMONTH(MIN('CalendarDate'[Date]), 0))
VAR DaysInWeek = COUNTROWS(VALUES('CalendarDate'[Date]))

RETURN
DIVIDE(MonthlyForecast, DaysInMonth, 0) * DaysInWeek

then a measure to calculate the quanity of orders each week: 

Weekly_Arrivals = 
CALCULATE(
SUM('Arrived Orders'[quantity_expected]),
FILTER(
'Arrived Orders',
'Arrived Orders'[expected_date] >= MIN('CalendarDate'[Date]) &&
'Arrived Orders'[expected_date] <= MAX('CalendarDate'[Date])
)
)

Then an initial inventory for each item at the start of the week: 

Initial_Inventory = 
CALCULATE(
SUM('Actual Inventory'[quantity_on_hand]),
FILTER(
'Actual Inventory',
'Actual Inventory'[item] = MAX('CalendarDate'[item]) &&
'Actual Inventory'[loc] = MAX('CalendarDate'[loc])
)
)

Then combined it to get the weekly forecast

Weekly_Forecasted_Inventory = 
VAR InitialInventory = [Initial_Inventory]
VAR WeeklyArrivals = [Weekly_Arrivals]
VAR WeeklySalesForecast = [Weekly_Sales_Forecast]

RETURN
InitialInventory + WeeklyArrivals - WeeklySalesForecast