I work in tree care, and have to sharpen saws often, because there is almost always metal in trees we remove from back yards. The only guide I use is the line that is scribed at the bottom of the link. As mentioned, it really helps if you can anchor the bar solidly, as in a vice. If I have to do the rakers, I do them first, so if I happen to slip and hit a tooth, it doesn't matter as much. Over time, you'll get better at maintaining your angles. Fresh files make a huge difference, and I rotate it slightly as I move it across the tooth. It's a matter of developing a feel for it with practice.