Psi: If you look at the link on my post on yesterdays training thread, you'd see where the Horwill recovery pace calculation comes from. As it stands I'm inclined to agree with you, but I'm gonna give the whole Horwill thing a try, hook, line & sinker, so to speak.
Drew: Good point on Roads vs. track, I'll bear that in mind, I also think Frank mentions what recovery HR is on his site, so I might work from that instead...