You might start with abductive reasoning which, in loose terms is described as guessing or "taking your best shot".
This Butte College summary of three types of reasoning (deductive, inductive, and adductive) has a fairly lenghty explanation of it, beginning with this:
Abductive reasoning typically begins with an incomplete set of observations and proceeds to the likeliest possible explanation for the set. Abductive reasoning yields the kind of daily decision-making that does its best with the information at hand, which often is incomplete.
Apparently, the single-word term abduction was coined by American philosopher Charles Peirce (sic) in his treatise "Abduction, Reason, and Science: Processes of Discovery and Explanation".
Peirce is also referred to as "the father of pragmatism", which brings to mind the word pragmatic. Pragmatic is defined in the Cambridge Dictionary Online as
solving problems in a sensible way that suits the conditions that really exist now, rather than obeying fixed theories, ideas, or rules
In loose terms, I might tell someone under the circumstances that you described, that I was being pragmatic, but I might just say that I will have to wing it, or, under the circumstances, it looks like we'll be freestylin' today".
Of course, those last two are not meant to be serious answers to your question, but they could be used.