Automatically inserting data into multiple cells from one spreadsheet after entering a no.

Hi everybody.
I've asked some questions here already and it helped me greatly, problem is that I've made an error of reasoning.
Let me start from the beginning.
I have an excel file which should contain all the items which are coming in and which are going out. I asked for a formula to help me keep track of the inventory and that it adds and subtracts whenever I enter someting in the 'outgoing' or 'incoming' spreadsheet. Thing is that I have different quantities when I get something and when I give something away.
Here's an example.
I get 3 packages with the item number 118, this means I have in total 300 clear folders(100 in each package) and now I give somebody 20 clear folders, when I enter 20 items with the item number 118 in 'outgoing' it deducts 20 packages from the inventory...
I don't know how I can solve this. Suggestions even if they are on a basic level would be appreciated.(Adding a spreadsheet, adding a column etc.)
It's this formula

=10+SUMIFS(Incoming!B:B,Incoming!C:C,'Writing utensils'!A2)-SUMIFS(Outgoing!B:B,Outgoing!C:C,'Writing utensils'!A2)

Another problem that I have is that I would like a formula which automatically fills in the blank cells in 'incoming' and 'outgoing' whenever I enter an item number.
I've highlighted 2 examples, one in each spreadsheet, showing you what I want to appear in those spreadsheets(yellow rows).

What's also important to keep in mind is that I have already a bunch of other formulas in place and I would appreciate if nothing stops working (except for the aforementioned formula)

=10+SUMIFS(Incoming!B:B,Incoming!C:C,'Writing utensils'!A2)-SUMIFS(Outgoing!B:B,Outgoing!C:C,'Writing utensils'!A2)
it's in the column 'Stock'within the spreadsheets 'Folder' to 'Misc.'