Could you expand on how this metric would be practically defined?

The problem isn’t lifting to C code, but rather “good C code”. For example you can do a 1-to-1 translation on each assembly instruction to C code that will do the same Machine state changes. This is not usually why you want, as it comes with a lot of extra cruft. When people think “decompiler” they think of n output that looks like what they would personal write. But that’s very Ill-defined. And, personally idk how one would define such a thing.