Use the rule that they formulate based on the case you are reading; that's why you read the cases. In some cases it will be very similar to the rules they use in reaching the decision. After all, that's what lawyers and judges do, they take a new set of facts and try to find an existing legal precedent to fit into those facts. But make sure you can pick out the specific rule based on the facts that are in front of you. That's what goes in your outline.
No, sorry for the confusion. You use the rule that they formulate after they arrive at the conclusion. But like I said, it maybe virtually identical to the preexisting rules you read in the case.