Bear in mind that more players use metric systems than imperial (according to minecraft server statistics, and the fact that there are few countries which still use the imperial system).
It is far easier for users to understand that half a bucket is 500mb, than it is for them to see 648 "atoms" (If that is the approach, I would prefer to use something like "voxellites" which has no real-world bearing than "atoms" which could cause huge confusion for users of chemical mods).
And to the next argument that you would just display it as "3 1/2 buckets" or 3 1/1296 buckets", that is hugely unnatural for me (and presumably many others).
I'm not sure what the issue with 144mb/ingot is. Why does a melted compound have to fit in the same space as a solid one? It doesn't in real life.
And as far as cauldrons, I think it's fair enough to just lose 1mb in a cauldron. Who cares about 1/1000 of a bucket of an infinite resource? And for those mods which have finite supplies of water, they usually add their own mechanisms for collecting/storing it, so the cauldron issue is moot.