Just an idea I'd like to float up here.
The idea is:
Each tile that borders only tiles of its own type and/or walls gives +1 to chamber upradability.
Each tile that borders a non-wall tile that is not its own type (including opens) gives -1.
If the total upgrade points is high enough, tiles in that chamber can upgrade to the next level.
This example reaches +14 and -4, the total is 10.
If the level 2 upgrade comes at 6, the level 1 tiles of this nursery can be upgraded.
If the level 3 upgrade comes at 12, the level 2 tiles cannot be upgraded.
To maintain the restriction of the need to have enough tiles of level 2 to make a level 3, a level 2 tile cannot upgrade as long as it is adjacent to a level 1 tile. Or it could check if there are more than x level 2 tiles in that chamber before allowing level 3 upgrades.
This makes it a bit more complicated than the current system used for tile upgradability, but it also gives more freedom to the shape chambers can have.
For example, a nursery that is 2 tiles wide and 15 tiles deep with bends in it would not be able to reach level 3 with the current upgrading system, as each tile has 2 walls (+2) and 4 level 2 tiles (+8) bordering it (total = +10, not enough for level 3 at +12).
Taking the other calculation method brings out: 2 tiles at the front for bordering disliked tiles (-2), 28 tiles in the back bordering liked tiles (+28). This gives us a total of +26, allowing the tiles in that chamber to reach their 3rd level.
The idea is:
Each tile that borders only tiles of its own type and/or walls gives +1 to chamber upradability.
Each tile that borders a non-wall tile that is not its own type (including opens) gives -1.
If the total upgrade points is high enough, tiles in that chamber can upgrade to the next level.
This example reaches +14 and -4, the total is 10.
If the level 2 upgrade comes at 6, the level 1 tiles of this nursery can be upgraded.
If the level 3 upgrade comes at 12, the level 2 tiles cannot be upgraded.
To maintain the restriction of the need to have enough tiles of level 2 to make a level 3, a level 2 tile cannot upgrade as long as it is adjacent to a level 1 tile. Or it could check if there are more than x level 2 tiles in that chamber before allowing level 3 upgrades.
This makes it a bit more complicated than the current system used for tile upgradability, but it also gives more freedom to the shape chambers can have.
For example, a nursery that is 2 tiles wide and 15 tiles deep with bends in it would not be able to reach level 3 with the current upgrading system, as each tile has 2 walls (+2) and 4 level 2 tiles (+8) bordering it (total = +10, not enough for level 3 at +12).
Taking the other calculation method brings out: 2 tiles at the front for bordering disliked tiles (-2), 28 tiles in the back bordering liked tiles (+28). This gives us a total of +26, allowing the tiles in that chamber to reach their 3rd level.