Friel has actually modified his recommendations since the printing of the book. He said something about "noodling around" in zone 1 not being effective use of valuable training time. It seams his more recent recommendation focuses a good amount of time at Aerobic Threshold (AeT) which he calculates at 20 beats below Anaerobic Threshold (AnT). I think for most people that puts you 1-3 beats from the top end of zone-2.
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Base Training Article- Friels Blog