My personal (unpopular in some circles) opinion is that 3-cushion systems are overrated as a learning tool for advanced players, and just about useless as a learning tool for not-too advanced students of the game. In fact, all this doing-math-in-your-head while you should be playing billiards will be counterproductive. There's no way a 0.4 - 0.7 player will be able to digest systems knowledge AND do all the real-life adjustments the tables force on him/her. More and more top players are freeing themselves of ballast, and are going back to playing on feel. If you are below 0.7, forget about systems, focus on choice of shot and technique.