Productivity isn't necessarily the wrong term here. In the context of individual productivity, your Input is always going to be a unit (say, a day) of your tim

Productivity isn't necessarily the wrong term here. In the context of individual productivity, your Input is always going to be a unit (say, a day) of your time. While you can split your time between a number of different tasks, your overall productivity still has 'one day' in the denominator (at least until cryogenic freezing is an option). Second, Output doesn't need to be measured in lines of code, it can be Value. The way you have written 'Value Delta' as a difference of two Values measured one day apart you have obscured the fact that time is in the denominator. The interesting quantity for a given interval is (Value Delta) / (Time Delta), which can reasonably be called efficiency or productivity.