I don't think it's actually ever stated in the books, but I tend to assume that that Superb (or whatever, based on thinner barrier etc.) difficulty probably doesn't get you very fine control over where you end up - but with a better roll you can pick spots more easily (there'd still need to be an association with the mortal-world location you're opening the portal from - but you might be able to choose among the possible linkages).
I treat control of where you end up as purely a matter of declarations. I don't think Thaum lets you choose, just like it doesn't let you choose which room you walk into when you walk into a door. Lore/Scholarship/Empathy declarations about affinity, history, or mood are all required to make declarations about where you'll go. I use the strength of that roll to determine how accurate you are.
For Worldwalking, I use a base strength of Superb+1 per each exchange they want to door to stay open+1 per each person they INTEND to go through. Increasing the size of the door is +2 to accommodate a vehicle sized object, +4 to accommodate truck sized object, etc. Certain forces from either side can oppose these rolls and add difficulty.
If it's used as a transportation method to other parts of the mundane world, I'm working on an accuracy chart, but if it's used to get to the Nevernever, I use the above. I'm thinking something like +2 to get to a specific continent, +4 to get to a specific country, +6 to get to a specific city, +8 to get to a specific neighborhood, +10 to get to a specific building. +12 to get to a specific room within a building.
I would also try to have a degree of travel safety...but now sure how to work that out. Instead, I just compel and make them deal. Again, the second method is for when it isn't used as a plot focused element, rather just a means of transportation.