AN fittings are 37deg flare, so if that is what you are plumbing the system with then you would probably be best picking up a 37deg tool. Jegs/Summit sell them pretty cheap. I think you can use a 45deg tool if you are in a jam, but I'd pick up or borrow the proper tool. Flaring can be difficult enough.
http://www.jegs.com/i/JEGS/555/80084/10002/-1