Are you still using the heart rate below 60 to figure out where to start?
I still use it sometimes, but most of my clients are already generally fit. It’s more so something to focus on with younger kids and less developed athletes.
I had a guy who was a “kind of out of shape” basketball player whose resting heart rate was about 88 bpm at the end of the season. He needed extensive training, so we did six sessions a week and two of them were purely cardiac output work.