You are utterly unlimited in introducing additional knowledge. It just has to be true, is all.

Hmmm. This makes finding the correct answer very tricky, since in order to be completely correct I have to factor in the entirety of, well, everything that is true.

The best I'd be able to practically manage is heuristics.

People refuse to employ this simple and elegant line of reasoning when figuring out what to do, as if a decision-making criterion must be nonnatural.

Other people are more alien than... well, than most people realise. I often find data to support this hypothesis.

Actually making the prediction is very hard, and what we practically end up doing is using heuristics that roughly guarantee, "I will not regret this decision too much, unless I gain sufficient additional knowledge to override almost everything I currently know."

I don't think it's possible to do better than heuristics, the only question is how good your heuristics are. And your heuristics are dependent on your knowledge; learning more, either through formal education or practical experience, will help to refine those heuristics.

Hmmm... which is a pretty good reason for further education.