>"Not too much, not too little, just right" doesn't exactly capture this cup situation but it is close.
>You want an adjoint Goldilocks principle.

In terms of the Goldilocks story, it would be something like:
Goldilocks walks into the house of bears that did not have any child (cub?) in this version and is forced to choose between food that is too hot or too cold; beds that are too stiff or too soft. There is no ideal, so Goldilocks must make do with whichever choices are the closest approximation to her ideal (maybe the stiff bed isn't as stiff as the soft bed is soft).