Fair enough, I wouldn't want to suggest you only go by a machine. Pretty much always, it's how you generally feel yourself. But we have avoided many asthma attacks (not sure if covid works the same way?) By seeing lung capacity reduce slowly first. My son would never have noticed himself. But one GP was right, when he said they hardly ever come out of the blue, as they seem to. There is nearly always 7-10 days of decreased capacity, before the patient even notices.
So, you can sort it, before it becomes urgent.