Where to shoot at largely depends on in which direction the other plane is moving. Apart from that the cal .50 mgs travel pretty fast, so if that is the question you need less lead than with slow guns.
Usual rule of thumb is to lead about double as much as you think you need. Definetly try that out, hits surprisingly well. So if you find yourself leading 3/4 of the gun circle diameter try to go for 1,5 of the diameter.
Get the lead you think is right, fire a burst, wait and see if it hit. Do not fire and then drag the tracers onto the target, wont hit that way.