theres a procedural problem with this, and its apparently why the feds dont require serial number on a PMF until FFL transfer is about to occur

when should you be required to serialize it?

if you serialize after it is worked to the point of being a firearm, then there is a period in time, however short, when the firearm is unserialized, thus illegal, thus serializing after creation could be obscuring a crime.

vs serializing before firearmhood, and you are now requireing a "hunk of metal" to be serialized because of what it MAY become in the future.

and just when does a hunk of metal start becoming a firearm, the so called 80% threshold

Not really. You can just register prior. If hunk of metal doesn't become a gun do nothing.