The word I'm looking for does not entirely have a positive connotation as it may mean that you don't have a certain style or attitude in making decisions. You may have just decided based on what is available or on what you know.
To Infer may fit.
- to derive by reasoning; conclude or judge from premises or evidence;
- from facts, circumstances, statements, to indicate or involve as a conclusion; to lead to.
- to guess; speculate; surmise.
- to hint; imply; suggest.
- to draw a conclusion, as by reasoning.
Inference is the corresponding noun.
- the act or process of inferring.
- something that is inferred.
- (Logic) the process of deriving the strict logical consequences of assumed premises; the process of arriving at some conclusion that, though it is not logically derivable from the assumed premises, possesses some degree of probability relative to the premises; a proposition reached by a process of inference.
An educated guess or a guesstimate. A ballpark estimate might work too.
Guesstimate is defined by Webster's Unabridged as "an estimate formed without adequate factual or statistical information".
Also, when you don't have enough information, you can go with your gut. Or make a gut decision. Or make a judgment call (when objective information is not available.) All of these can imply lack of data and can fit in context.
In the field of artificial intelligence, such methods of problem solving on the basis of incomplete information and with the help of loose rules of thumb are described as "heuristic". A "heuristic" is defined by wiktionary.org as a method that "solves a problem more quickly but is not certain to arrive at an optimal solution".
An option might be guesstimate:
to estimate without substantial basis in facts or statistics
However, this doesn't really convey that you have "some information but not enough", so it may not be suitable, depending on the situation. Perhaps the phrase educated guess might work better:
a guess based on knowledge and experience, making it more likely to be correct
Of the two, I would say guesstimate is the more derogatory, as it implies less of a basis in knowledge/information.
Definitions courtesy of dictionary.reference.com
SWAG. Scientific Wild Ass Guess. Technically an acronym. A popular term frequently used by military.
Try looking at "bounded rationality" in Wikipedia. Might work.