this is also known as the speed/ sucker rule. It is always best to teach ones self how to do the complex math of poker without shortcuts and practice it because in the future when you are trying to calculate your fold equity in a threeway pot with a K high flush draw the math can get pretty complex. If you plan on playing for entertainment or make a living at micro, then sure, use the 2-4 rule and forget everything I just said. Otherwise, it's best to try to learn the simple/ basic algebra that is involved in poker that will help you to calculate more complex situations in the future. Most poker books
have these simple equations in them, or you can always back order Bluff magazine because it has multiple articles written by the pro's about poker algebra. Hope this helps you. Good luck in your future poker endeavors and remember, if you start out learning simple things you will end up being a simple poker player, and complexity is the key to success in today's poker arena.