How to combine data across levels using demantra client expression

@production

My levels are

Item-->Production Line

Users enter data @production line level. But i need to combine the data across item for availability calculations

For example

Item      Production LIne     Demand

ABC     L1                             100

ABC     L2                              100

before even committing the changes to the system i need to show the Global demand is 200 for item ABC, as this is used for different RTSOP capacity  calculations

I was planning to create a series called "Global Demand" that totals the demand @item level even though the worksheet is @item, production line. How is this possible

Thanks

Gerald