“ basal is right if you remain steady at night, which she does, but it doesn’t say at what level“
The Clif Notes version is that the job of basal insulin (any/all kinds) is to keep BG steady; changing BG up or down is a job for fast-acting insulin.
Steady at night (at any number) is an awesome thing, because it means changing that number is simple enough with fast-acting insulin… just do a correction dose to get from say, 150 down to 100, and since her overnights are pretty steady, she should then stay at 100 the rest of the night.
Increasing basal much higher would mean she stopped being steady, and while she might wake up at 80, starting at the same 150 means a decent drop throughout the night (even though both numbers are “fine”) which probably means theres a similar drop during the day but we usually cover those up with food & exercise & adjusting carb ratios so they’re a lot harder to tease out (and thats why when using MDI they tell you to use the night numbers for checking basals). Although its easy enough to add carbs or fast-acting insulin to fix daytime challenges without even noticing, eventually if the basal is too high (or low) the carb ratios get adjusted to make up for it in the other direction (too low or too high) and the correction rates get bungled accordingly. While they can all still be used to stay in range, when something out of the ordinary happens its a lot harder to be quick and nimble if all of the rates and ratios and doses are a tangled web of compensating for an ‘incorrect’ basal setting.
For sure thats the simplified view; throw in some pizza, hormones, a possible dawn phenomenon, allergies or Monday morning stress and the neat & tidy logic gets a little shaky, but to the extent possible, basal insulin doses should just keep nights flat & steady, and if the level at which flat & steady occurs needs changing make that adjustment with fast-acting insulin (or carbs).