I don't think a culture take-over can ever become 100%. Because the domestic culture points will still be present. Like if the English have 100 culture on a tile, and the Irish manage to pump in 1 million culture. Irish culture will be something like 99,99% - but it still won't be 100%. It never will be, because the culture percent isn't rounded up.
So 100% culture condition on a foreign tile is flawed.
So 100% culture condition on a foreign tile is flawed.