A combination of all of the above.
Island maps (like the classic map) are different to maps with large land masses, as usually one those, you can be attacked from as many continents as you have borders.
On a single land mass map like yours, or Germany, often you could have 4 or 5 borders, but only be attacked from 2 'continents' or vica vera. That really affects how easy it is to get an hold a continent, and of course the overall position in a map.
I'm not sure there's any real formular other than making a rough guess, and then seeing if any adjustments need to be made after playing.