Resting heart rate actually is capable of fluctuating, like in response to overtraining or illness.
Daily Rest HR fluctuation isn't actually a symptom of overtraining unless you're talking about fluctuations in a very short timeframe like seconds (heart palpitations).
You can easily get same levels of daily Rest HR fluctuations in the perfect workout plan without overtraining.
Overtraining is much more closely correlated to your "Baseline Rest HR" which for practical purposes is treated as a fixed value, not fluctuating, not like a moving target that needs to be tracked down with precision. Not like that.
To avoid overtraining, you simply need to get your Rest HR dip to the Baseline Rest HR at least once a week. It doesn't need to be daily.
Because in a good weekly training plan/structure, there would be high training load days (to drive the physiological and metabolic adaptations) and light training load days for recovery. The Rest HR would obviously be higher during the high workload days and lower during the recovery days.
To achieve a near-constant Rest HR every morning close or at the baseline Rest HR, everyday is really not a good goal. In order to do that, you should avoid any exercise above Zone 1 intensity (recovery intensity) and keep your workout sessions under 45 minutes.
This is not a good goal because your exercise performance will quickly hit a plateau. You'd want your exercise performance to get better so your workouts will eventually feel easier or you can do higher intensities, burn more calories without burning out.
Because if your exercise performance isn't improving, your workouts will always feel hard and feel drained of energy after your sessions. Worst case scenario, you may even lose all motivation to exercise if exercise feels so hard every time.
Most smart phones now days can determine your HRV using the camera and flash on the phone. Like resting heart rate, you have to track it over time, however.
It's not as accurate, not even remotely close as the bulky digital blood pressure device my mom is using. I've once determined its accuracy by manually counting my pulse by feel in one minute. The bp device on the other hand is dead accurate.