The only metric that matters is "can you do with it what you want to do?". Readiness cannot be meaningfully projected onto a simple number, it's always a question of how your needs and the compiler and ecosystem fulfill those needs. For example most people say that Perl 5 is production ready, but it's still pretty much useless for number crunching/high performance computing -- so it's not ready for this particular use case. So, what readiness number would you give Perl 5, even if it's not ready for every possible use case?
Tracking implemented features is useful, because people want to know if they can use a particular feature. But doing that as a percentage of a pretty much arbitrary goal isn't very useful, because the current usefulness isn't really a function of future plans.
The same holds true for the tests: if we add or remove tests without changing the compiler, the readiness metric changes.
In the end the best thing to do is probably to build cool stuff with Perl 6 (or improve the compiler) and talk about it, instead of trying to come up with complicated (and yet still not very good) ways to measure progress.