I'm not perfectly sure if this is the question you're getting at, but I think you're asking, "Under what conditions can an alien bite a human in the head."
Now the thing with the aliens is that you hit where you're aiming on the human when you strike, generally, although the hitboxes in Tremulous are literally boxes so under certain conditions (being too close) you've gotta adjust more than that first statement would suggest.
So you can actually bite the human in the head from the ground with a dretch and anything above that (but this is obvious considering the size of the mara, goon, and rant). It is easier to headbite humans from higher ground though, especially from the walls.
There are lots of numbers out there for how light armor and a helmet decreases damage, but I won't go into that, and will simply say that it's enough to encourage that you buy it at S2 and even S3 whenever possible (if it goes better with the weapon than the battlesuit does) under just about every set of conditions. Also, know that there are no headshots for mara zap. And that advanced dragoon snipe damage is really weird but effective in any case.