Again, you have to define the alternatives. APIs have launch success criteria too. One way to assess zero-to-one initiatives, if that is what you are asking about, is to start measuring before the project is complete. For a UI, that could be as soon as it is usable but before all the features are implemented. Then you will be able to able to estimate the value of the remaining features.

You might like to read https://hubbardresearch.com/publications/how-to-measure-anyt...