Yeah. Presumably all of the resources in Dwarf Fortress are some form of claymation version of their original selves, so partial resources and requisite storage of them would definitely make some sense.
I mean, a dwarf takes the same amount of rock to make a door as to make an earring or other trinket? I doubt it. Highly.
If we get more RAW access, though, we'll definitely see a bunch of mods revolving around "resource realism"... Where lots of crafts will be partial-unit creations as opposed to a full unit to make a single item.
Another way we could handle it is simply look at item weight. Input weight - output weight = remaining resource weight. If resources are handled by weight and, presumably, all like-mineral units are the same weight... It would definitely simplify it beyond making an arbitrary "amount" that each unit is comprised of.
It could also be further said that a legendary stonecrafter is more adept at not wasting materials so as where a novice or dabbling stonecrafter would have an input weight - output weight - waste % = remainder... As they get more skilled, the waste percentage amount reduces leaving more useable resource weight. Any remainder underneath the lowest resource requirement could be discarded or, possibly, cut into studs or crushed into powder for colourings and such in paints. Or it could just be treated as claymation resources and be used as a partial resource for making something else.
This way we'd see a logical use of resources where the creation's weight is directly related to the amount of material required to produce it. No arbitrary or x/7 mysteries about it.
Also, I think the reason for x/7 is so it's easily expressable in hex. But that's just my crazy theory.