If you have the mics and time, place four mics of the same kind close to each other, the first one pointing to the middle of the speaker, the other ones positioned gradually towards the outside. Then listen to the four signals via your mixing desk's solo button. This way it's easier to find a starting position for the sweet spot. I find it very hard to judge the sounds when moving the mic around. After a few seconds I'm not able anymore to say if it's good or bad. With the method mentioned you can switch between positions and listen to them on your monitor speakers without crawling around in front of your speaker wearing headphones. Or having someone else do it for you. A good way if you're recording alone. But not everybody owns four SM 57, I know.