Last gen was different than this gen though for a few reasons. I think it'd be more fair to compare how games are selling on current gen, as I believe the western market is more aware or receptive towards such titles this time around.

some examples for North America, where the bulk of X1 sales are ~

Dark Souls 3 (PS4) - 450k

Dark Souls 3 (X1) - 160k

Roughly 2.8:1 sales ratio.

MGSV (PS4) - 1040k

MGSV (X1) - 360k

2.9:1 sales ratio

FF Type-0 (PS4) - 360k

FF Type-0 (X1) - 150k

2.4:1 Sales ratio 

So if we can imagine that KH3 will sell equal to or better than 3:1, Sony needs to sell around 3 million copies in North America. I think that is achievable with what the install base will be by the time the game launches. The caveat to that is that these numbers don't consider the Uk, another strong territory for X1 which I think can at least do 100k which leaves 900k for NA, which assuming the 3:1 ratio for Japanese games, means around 2.7 million in NA for Sony. Suddenly we start to reach the realm of possibilities. I also expect Kh3 to do better than 3:1 sales wise on Xbox. Perhaps a 2.6:1 ratio seems like a good amount.

Where you get the type 0 numbers from?

NPD had Dark Souls 3 and MGSV roughly 3:1. Which is close to what you have.

But NPD had FF Type-0 HD at a 4:1 ratio. The Xbox One version did so poorly that a port may not have even been worth it. Though I have no idea how much it costs to port.