I agree that the pluperfect is best here, though I'd put the preposition at the end. If you were to rewrite it in the present tense, talking about the current situation you might get
"If I knew that, I would be able to make an educated decision."
Here you have a past subjunctive then a present conditional.
Now if you want it in the past, the 'knowing' must still occur before the 'being able to,' so you have.
"If I had known that at the time, I would have been able to make an educated decision."
Here you have a pluperfect and a conditional perfect, so they still have the correct sequence of tenses. I am not aware of a pluperfect subjunctive in English, so you have to settle for the indicative (correct me if I'm wrong).