The cost to a business of lost developer productivity is not the salary cost of the developer doing non dev work. It is the lost productivity of the developer

The cost to a business of lost developer productivity is not the salary cost of the developer doing non dev work. It is the lost productivity of the developer not making the things you’re paying them for. The reason the business is willing to spend $x a year on the developer is because the developer produces >$x a year of value, in the form of revenue generating software. If you divert 5% of your time into doing something else, you are taking money the business wanted to invest into capital (new software features) and spending it on… something else. Lunch orders or whatever. Not making capital, anyhow.