Hm. Interesting topic.
An isolated thought-
One (admittedly influenced by another thread) is that humanity is not a thing upon which we can put a numeric value. Even just to be demonstrative, I'm not sure it's helpful. But, as I said, I'm being influenced by another thread, so don't worry too much about that...
Now, about aliens....
It's true that an author would have great difficulty depicting truly alien creatures. If the mentation and motives of a character are so odd as to be unintelligible by humans, the author really cannot get "inside it's head", and show you what is going on. Such creatures become forces of nature. You may be able to define rules upon which they operate, but winding out the underlying mechanic to the rule becomes difficult if not impossible. Such aliens are... well, they are like the weather. Frequently we can tell what will happen, but we know somewhat less about why it happens
Orson Scott Card addresses this somewhat in the Ender books. He has terms (stolen from Icelandic, I think) for different degrees of "alienness": framling, raman, and varelse.
The framling is one of your own people. You can understand them fully, and dealing with them is easy. We here on the boards are generally framlings to each other.
The raman is someone who is not of your own people, but is still comprehensible. It may take a little bit of work, but you can get into a raman's head, understand their needs and thoughts, and deal with them cogently.
The varelse is truly alien. So alien that no communication is possible. There is no abilty to understand and negotiate, or compromise. The inability to communicate is so complete that the alien might well be raman for all you know, but you cannot tell, because you don't understand anythign that they do.
This is seen in
Ender's Game. The "buggers" are varelse. Humans cannot understand or negotiate. There is conflict, it turns bloody, and funally genocidal. It is only after the war is over that Ender discovers that the buggers were actually raman.
As to how alien some aliens are...
Well, here's the thing - it takes a lot of time and effort to make an alien deep and understandable. If there's a lot fo differences from humans, you will at first misunderstand most of it, and characterize the creature by the strongest features you do see - leading to the archetypes we see in D&D races, for example.
A good example of this are the Kzinti of Larry Niven's "Known Space". In most of Niven's own works, in which kzinti appear, but aren't the central characters, the kzinti are a characature "warrior/predator" race. However, when you specifically read the more in-depth treatment given to the kzinti in the Man-Kzin War series (a series in which Niven opened things up as a "shared world"), the kzinti are far more fully developed. They are alien, make no mistake, but rather than be a simple charicature, they are shown to have depth and variability and complexity. But it takes a lot of verbiage to get al that across...