How many units of thinking did you perform today? How do you charge or how are you payed per thinking unit?
The goal is to measure people's results. It might occasionally take a relatively-long time to find a seemingly-simple solution. But if an engineer /always/ takes a long time to find every solution, and upon inspection the problems were not actually difficult, then you most likely have a low-productivity engineer on your hands.
The company as a whole makes a profit, how do you know what it would have been if feature X wasn't implemented? And ten people had a role in deciding to implement it, four in implementing it.
How can I possibly put a number on my contribution?
I remember once being a similar conversation regarding a training program I was working on, and the customer was trying to assess the value of the training not on the improved performance of the employees, but on some measurable "knowledge unit" that had been transferred to the student (regardless of their ability to retain it).
It was beyond frustrating.
Your number won't be exact, but it doesn't have to be and double counting among the other people isn't always bad.
That I work on the front ends was only partly my choice, we all move around depending on where we need people at that time, and other people decide what needs to be implemented.
If we were to fire all developers, we would probably lose hardly any sales the first year. But over time, licenses would go down more and more. So part of this year's profit is due to the work of people who left us years ago.
I don't see any point in claiming some specific part of our profit. People would laugh at me, probably.