Why are you calling me a, GOPHER...lol?
The average is computed by X2.
i.e., if you shoot 30 consistently, then X2 of that to make up for the fact that there are only 9 holes. This would put your average at 60. This is a very simplistic answer but you get the idea.
It is weighted and calculated the same way a real handicap is calculated.
nivlac and tibbets have answered this much more in depth in past posts. They may copy that here.