A company with multiple activities must read the shared costs without assigning to the breakdowns a precision they do not possess. Rent, administration and common tools can be distributed according to different criteria. Each criterion illuminates one aspect and can leave other elements out of comparison.
I propose to distinguish the contribution before the common costs from the result obtained after their allocation. This distinction helps when evaluating a specific change: closing a task does not automatically eliminate the share of cost that was assigned to it.
A hypothetical example concerns two services that use the same office. One requires more staff hours, the other more space. Repartitioning each cost based on turnover could be bad for both. The criterion should follow, when possible, the way in which the resource is used.
The final reading includes the actual modifiable costs and the consequences on other activities. Making assumptions visible allows for a more honest comparison. A breakdown serves management when it helps to discuss concrete decisions, without turning into an indisputable measure of the value produced.