How are you supposed to calculate overhead expenses?
Estimating overhead expenses by applying a coefficient to salaries can make sense in some circumstances.  In an organisation which is mainly office-based, overheads may consist mainly of the costs of office space, office running costs (electricity, cleaning, etc), computer equipment and support, and (depending on the local jurisdiction) payroll-related and property-related taxes. Using a fixed coefficient will always be an approximation to actual costs, but sometimes it can be a fairly good one, especially if the organisation has a fairly "flat" structure with most staff being on similar salaries and having similar office and computing needs.

In a manufacturing organisation with much capital equipment this approach would be inappropriate.