I have a list of sub-items on my board.
Each sub-item has its own Income/Outcome amount and there’s also column is I/O Type classifying the Income/Outcome as Outcome or Income.
Since I understand that there’s is no out of the box option to make a dashboard based directly sub-items,
I’d like to have a formula on the parent line that will show the only the sum of the outcome lines.
Welcome to the community! You can’t do such a thing with formulas (as you already discovered) and summing the subitems to a parent has 2 disadvantages in your case:
the sum will add ALL subitem values (no filtering)
the sum in the parent will be a mirror type (not a number) and can’t be used in a dashboard