Natural eye contact in conversation, and in presentations for that matter, is about four seconds. It's about a complete thought. Measure it when you're having conversations with friends. Do it tonight. Look at how long they look at you before they look away. When you're presenting, keep in mind that if you're talking you should be talking to a human being. Not to your notes, not to the wall, not to anything but a person. If you're not looking at a human being because you have to look at your watch or such, then break eye contact, be silent, be relaxed, look at whatever it is you have to look at and then regather your thoughts. Look back at a person in the audience and begin again. Now this can be difficult, a lot of people have difficulty making eye contact at all. The trick here is to work at it. Push yourself to give yourself, to give the person you're looking at one extra seconds worth of eye contact. After that point when you think you have to look away. Look, in other words, look at them as soon you feel like you have to look away, push yourself to look for a second longer. Over a period of days you can get to the point of giving four seconds worth of eye contact. Don't ever worry about staring people down, you'll know if you're staring people down because they're, they start to back away or they look away. But generally, you won't be staring people down with natural eye contact. And if you give natural eye contact in your presentations you will keep your audience engaged with you.