Peter Breu replies: This is a complicated topic and of course in part depends on what sort of jointer you have. I would first turn to Fine Woodworking magazine to read about the subject. There are also whole books on such subjects—Jointers and Planer by Rick Peters, Sterling Press is one example.
Jon Siegel replies: The knives should be equal to each other within 0.002″. Also, the knives should be parallel to the outfeed table within 0.002″. And finally, if the work “snipes” at the end, the outfeed table is too low. But if the work “rides up”, the outfeed table is too high. On average the best position for the outfeed table is 0.002″ or 0.003″ lower than the cutting circle.