Lemme dumb down the setting of MoN so we can clear up the confusion.
While the inside of the Dangai is not literally outer space, and the realms aren't planets, you can think of the two realms being pulled together in the Dangai, as two planets being pulled together in outer space. It is as simple as that.
I ain't just gonna take your word for that. Although I personally also really don't care about those rankings.
So let me just drop my PoV for all scenarios here and then people who know and care can either take it or ignore it:
If they are part of the same space that means they are part of the same universe. If they are part of the same universe, merely containing stars and galaxies wouldn't qualify them for being universe sized. A
galaxy cluster also contains those things and isn't universe-sized either. We only assume so for dimensions that are spatially separated from the normal universe, instead of just being a small part of it. So the size should probably be rechecked in that case.
Aside from that, in that case, all interdimensional rankings would need to be removed.
On the upside, the calculation method in and of itself would be fine then.
If they are not part of the same space then normal gravity, which can only "travel" through 3 spatial dimensions, couldn't pull them together. That's simply since by travelling in the regular 3 directions we have the gravity of Earth wouldn't be allowed to reach Soul Society. Instead you would need a gravity that can go in more directions, like going through an extra direction offered by Dangai. Four directions would mean we aren't dealing with normal gravity anymore, though. The inverse square law, which our normal gravity obeys since it spreads through 3 directions/dimensions, wouldn't hold for something that goes through 4. What this means is that calcing this feat like that wouldn't work.
Now there's also the question of whether moving universes, or universe-sized dimensions, is a universal feat. That is probably somewhat case by case. Generally, if you can launch something so hard that it would destroy the universe you should be good to go. In this case, it seems to be more opposing a (potentially increasing) force that is pulling them together and cancelling that out doesn't transfer universe destroying amount of energies on the dimensions and the top-tiers seem to scale above the transferred energies more so than the total, in my understanding.
Sooooo idk.