First off I have to say I've been in similar situations, you just have to learn from them. I'd also like to say I'm still young, 20 years old and I open carry every day. That being said I think pulling your gun on someone wouldbe justified if that guy had driven by, and just jumped out and started charging you. With you gesturing anything it could be taken in the wrong context and make him flip out.
I think every situation is different, If a person tried to attack me with any weapon from a butter knife to a missle I think I would draw my weapon, hopefully that would knock some sense into the individual. If not I think you can "assume" that persons intentions are nowhere near good and you are forced to defend yourself accordingly.
Now if a person was attempting to attack me unarmed I'm not quite sure what I would do, I think it would depend on the situation, size of the person, and I guess my split seccond judgment on what to do. Hopefully pulling out a gun would not be needed. Hard to make a real decision while calmly sitting on the computer.