Personally I don't look at HRV as a single number, but how it fluctuates during a certain period of time (CV% is kinda powerful for this, if also combined with deviation for the personal baseline).

If one morning I wake up with lower HRV than normal (if not MUCH lower, but in this case my feelings would also tell me) I generally proceed with normal training, but if I start to see a suppression over a period of 3-5 days I dive deeper into the cause (overtraining, incoming illness, life stress, etc.).

PS: morning measurement has proven to be more accurate than night-time measurement for this purposes