I’ve been using HRV4Training. I feel like the numbers are training me … to get more sleep, to not drink alcohol, eat the right foods. 
I can usually feel when I’m tired and the HRV number confirms it.
Monday was a good example of the reverse. My HRV was on the low side and I should have taken it easy. I didn’t do intervals but I rode outside on gravel for 2 hours which left me good and tired. The next day I was even more tired and today I’m still tired and feel like I’m getting sick. I should have listened to the HRV on Monday.
So I’m pulling the plug on this week and it will be a rest week. I’ll do my group ride on Saturday but try and stay out of the red and not take pulls or contest sprints. Sunday will be a full rest day. Hopefully I’ll be back in action next week.