Mel,
I know what a conumdrum you are in, why write darker when most of the time they are a smidge different ( but which way). What you have on your side is that if tomorrow it is lighter then it was to be calculated from yesterday, if it is darker they can take it as O day. I suppose sometimes it is better than thinking have we missed O all together which we have up as our advantage. Are they going BT you tomorrow?
I was always told similar was O, but you will find out in the morning or even if you were to do one tonight..it should be fading
Bec xx