Planet specialisations - what are the tresholds?

[Stadtpark]

I'm trying to add Exotic Gas Refineries here and there, but I don't want them to change the planet type to "Refinery World" on most planets where I add some.

Any idea what the treshold is? Can I build 3 on a planet without having it's type flipped to refinery world?

Any idea what the game mechanics are for deciding when a world becomes specialized and how concurrent situations are resolved / weighed?

As I'm on an Ironman game, I'm a bit reluctant to just try stuff out.

Edit: I just built a 3rd one, and it didn't switch: it is still a mining world - size 14 planet, 7 mining districts and a Mineral Purification Hub on it, and now 3 Exotic Gas Refineries: net mineral production went from 131 to 121 because now 10 minerals more are directly converted to exotic gas, just as intended.

 

[PeterPancake]

https://stellaris.paradoxwikis.com/Planetary_management#Planet_specialization

Depends and early world can switch easily.
Rule of thumb for that specific:
Every 4th non-upgraded Alloy, Research, CG you can have safely 3 Refineries.
Every 3rd specific district (or +% Building) is 1 refinery. (Agri, Mining, Energy)
Upgrades decrease the lever from refineries further.

Not so important but:
Every 4th housing districts is 1 refinery. (Urban)