I don't have the maths right to hand, but burning to a jool orbit with low periapse is indeed the most efficient way because you get a much biger boost from the much vaunted oberth thingy-ma-bob. But in practice, unless you've shaved your fuel allowance very finely indeed, it's often easier to just burn away from Laythe directly, because you don't have to wait until Laythe is in the correct position in its orbit around Jool.