What is the ratio of good versus obsolete code overall? (what % of total is unused?)

How does that ratio compare with other projects? (eg the Linux kernel, various distributions, even MS Office etc. etc.)

I'm wondering if there's some sort of constant, or commonality that could aid the removal of unused code regardless of the project.

And how much of this sort of thing, having a lot of outdated, unused code, is a matter of documentation keeping up with development?