using a total calculation in a filtered portal in a calculation

[fiunary]

Hi, I am new to this. I want to track stock and have a portal on my product page which shows invoice line items relating to a given product. The qualities in each of these line items are summed (via another filtered portal from a sum calculation in line items).

I have an opening stock for each product and I want to subtract the total sold (from line item portal) to this. Is this possible?

Many thanks,

Alex

[fiunary]

Not sure why but the calculation automatically included the filter so it works with no coding.