I use the HRR formula to set my heart zones. But I don't use these for training purposes on the bike.

On the bike I use a very expensive measuring device called a power meter, which measures exactly how much effort I'm putting out. That's what I use for training on the bike. It has a strain gauge (like a bathroom scale) in each pedal that knows how hard I'm pushing, and motion sensors to know how quickly I'm turning the pedals, and together that data is even better than heart rate.

But that doesn't work running, swimming, or climbing. So heart rate reserve it is. And even on the bike, I use the %HRR zones to evaluate the quality of a workout in cardiovascular terms.