I am not sure if the rules dont allow wrc cars to put mirrors more out, but they use like this:
At the moment distance to sideglas is 90mm and shortest distance to mirror 50mm. I think the place has also little bit effect where the mirror is. Like in the volvo its rigth on the same level as the hood so there is more air going there than little bit lower.
I did not want to make long leg as it will create more torque and does not look so nice anymore. Best option would be like the huyndai that the part is thicker on the end which is connected to car but then it fits only to one car model.