Thanks for writing in with your question. It s a good one. Let me give you some background:
When Steve and I wrote that book we were pretty sure that no one would really care. Alpinists and mountaineers did not train. They just went climbing. The better ones went climbing a lot. We knew we had to present them with the simplest way to figure their zones. So we used the percentage of maxHR you have found. However this is the least accurate way to find your personal zones.
Fast forward a couple of years and found out that our book and ideas were gaining a lot of traction and that climbers really did want to take these ideas to heart. So we showed more effective ways to calculate the zones.
There is a wealth of information to help you train better on this site.