It sounds more and more to me like you're trying to find a definition of what Lydiard called best aerobic effort. Much like Double, I tend to just cruise and settle into what feels right. With experience, most people tend to know where this is. If you're unsure, that usually means you need to slow down.
Personally, this pace usually falls pretty well in line with 2 minutes per mile slower than 5K pace, the pace mentioned by denton.