Being able to confidently predict "it looks like he will have a cold in one or two days, let's increase his dose and preempt it" based on PEF (peak expiratory flow) measurement done morning and evening was such a boon. Also being able to confidently identify patterns like "he always gets worse when we do X". We did it with pen and paper, but today of course there's an app for that:
By the second child i was too tired to keep stats, so we winged it and the docs and I happily acknowledge that ignorance is bliss (sarcasm, i think).
(You’ll note, not for being wrong, the diagnosis got independently confirmed both times.)
Medical professionals like and are used to being the unquestioned experts. But you are always the expert of your own child (because, like I said before, every child is different.)