My father was Canadian-born but originally of English descent. My mother was pure 日本人。 I was actually born with blue eyes, but now I have green ones.
I don't think there is a particular formula for how Asian or white a child looks depending on which parent was which, although I've heard people claim that when the mother is the non-caucasian the child looks more caucasian vs. the opposite case. I've met a few 'hafu' myself and I can always tell they're 'something' but its never really clear precisely what until we exchange introductions and ethnicities.
Half-asians (in my opinion) look a little like those from the countries bordering China, like Kyrgyzstan and Kazakhstan:
(Please ignore Borat in the middle hehe.) If we look at this 'typical' presentation of Khazakhs they possess some of the features of an 'Asian' person but do not fully resemble someone from Japan, Korea, China, etc. The skin is not as fair, the eyes are not as narrow, etc. And of course, there are people from China who wouldn't fit that description either, I'm clearly speaking in generalities here.
The premier of Khazakhstan:
A Kyrgyztan bride:
I've also heard outsiders say that half-Asians resemble Mauri and/or Samoan individuals.
I think it really comes down to outside perception, which in turn is based on a person's education and level of exposure and familiarity with various ethnocultural groups and their dominant characteristics. The varying level of 'Asian' resemblance dictates what group an individual will most likely look similar to, however, a persons ability to pinpoint what exactly that is will be determined by how familiar they are with the who's who of the world. I know some people from small town Canada who'd simply call everything from native Aboriginal to Philipino to Mauri 'Chinese'. I mean look at the southern states of America following 9/11, people were chasing Sikhs with baseball bats for God's sake.
But genetics vary too greatly to say whether there's an inherent pattern. Political correctness usually negates the discussion altogether. One thing that's for certain, the white kids don't consider you white and the Japanese definitely consider you gaijin. Hafus are a group unto themselves.