Thanks, Jimmy. Yes I know about the variations in HR and using the monitor primarily as a max warning.
An aside on the value of HR monitors. I rode the Lolo Pass ride seven years ago (when all of my numbers were respectably higher!). A century with several tough climbs going out, then Lolo Pass and two climbs coming home tougher than the ones going out. I set my limit at 154 on the early climbs. Couldn't stay with any hotshots at that RPM. For Lolo Pass I stayed at 164 and felt good. Paid no attention to the monitor coming home, but had enough in the tank for the climbs.
Oh, on the pass, I shelled a rider 15 years my junior who I'd been pacing a mile from the top. He thanked me later, saying he was hurting and I got him up the climb far faster than he would have done on his own. And I passed a lot of cooked hotshots coming home!