Bahhhh
I made all the calculations, but i missed some...
I'm close to the dom limit, and there's nothing i can do to avoid it before the end.
But i've learned this: a city on the 3rd level of culture is 35 tiles, equal to 1.84% in a large map with 60%.
So, before to take a city, you must consider this, since in a short time it reaches this level.
Next expansion has to be considered if it's a border city, otherwise it doesn't count much.
To be safe, you can add this value to the % you already own, plus some % for the high culture expansions here and there.
Another thing i've learned is that found a corporation which competes with one already present, removes this one, so if you found corps only for wonder score, you must do it where it has less impact.
Maybe i'll try one last time.