Over time, West Arete has developed a convention for how we organize our spec directories. This arrangement has been very successful for us over many years and many projects. We'd love to extract it into some kind of framework/gem, but have never managed/invested to do so. So at the very least we can share this writeup of what we've found to be useful, in the hope that it's useful to others.
The top level of our spec directory looks like this: