[gentleman11]

Yet, if you had to build the aircraft out of metal shavings one sliver at a time in a laborious, error prone process, you might not know how far along you are but “well, we’re halfway there by mass” at least gives some measure for the project as a whole. If you game any metric it becomes trash

[detaro]

That assumes you have a finished plane design and know the target weight, which I don't think is what the analogy is after.