Gary, great explanation!
Suzi, the Polar website explains this in depth. Here's a link:
https://www.polarusa.com/targetzone/default.asp for figuring mhr (max heart rate) out. Another good site is:
https://www.bodyforlife2.com/max_heart_rate.htm
Basically, you cannot rely on just the 'formula' as every body is different. It is a good starting point but, I found, if I used it only I'd be moving way less. I hope this helps.
