I have over 2000 daily HRV readings. I take them with Elite HRV on my phone and my Polar watch, just to check and because the watch gives me an accurate number for resting-standing minus resting-lying. For some reason, with age my morning resting HR has become mostly useless as a check, so I added HRV. I'd give it a C for usefulness, but I do it anyway. TSB is a better guide for me. One thing the Elite app has done very well is to predict the onset of PMR, twice. When my parasympathetic power goes in the toilet, I'm about to have a problem. Otherwise it's rather a crapshoot. Of course that's just me. I'm probably not a good case study.