Welcemir

That's the neat part, the scheme does not even need a boundary, much less a straight one. It does require 2 blocks clearance above, since you need to be able to move between the holes. If the layer to relieve blocks from does not stretch to the mantle, it is probably faster to mine each shaft with X completely with the adjacent blocks marked "-" every second layer of blocks until the stone layer ends and then mine towards a central shaft to get back to the top of the layer and then mine the next shaft. It will take a lot of durability from the pickaxe initially, though, will only start to relieve blocks consistently with the 4th shaft. After that initial investment, the efficiency approach 40% of all blocks you remove

Very true, especially because I am mining chalk to get those white stonebricks. I will need the chalk stones for mortar and leathermaking anyway.

Could you elaborate how to do that exactly? I cannot come up with something that would get closer to 50%

Possibly the simplest and fastest scheme out there (but only 1/3 efficient):

Sorry, I had an error in the previous answer, the scheme posted by OP is less efficient as I initially assumed, because it requires you to break one whole layer of (N-2)x(N-2) blocks. About the 40% efficient tileable scheme I had in mind (view from top, the repeating unit is lined out): Layout A: | Layout B: | Legend: ____X_________X____ | ____X_________X____ | X...Break these blocks (line up between layers) |X - + - + X + - + -|X | |X + - + - X - + - +|X | -...Break these blocks |- + - X - + - + X +| | |+ - + X + - + - X -| | +...These blocks are relieved and drop when the layer below is finished |+ X + - + - X - + -| | |- X - + - + X + - +| | X|- + - + X + - + - X| | X|+ - + - X - + - + X| | |+ - X - + - + X + -| | |- + X + - + - X - +| | |X + - + - X - + - +|X | |X - + - + X + - + -|X | |+ - + X + - + - X -| | |- + - X - + - + X +| | |- X - + - + X + - +| | |+ X + - + - X - + -| | X|+ - + - X - + - + X| | X|- + - + X + - + - X| | |-_+_X_+_-_+_-_X_-_+| | |+_-_X_-_+_-_+_X_+_-| | X X | X X | Step 0: Mine out a 2 block high space above the area you want to quarry so you can see the ceiling Step 1: Break blocks marked with X and - in the top layer (layout A) Step 2: Break blocks marked with X and - in the next layer (layout B). Blocks marked with + in the top layer (layout A) are relieved and drop The top layer (layout A) is now gone and dropped 40% of all blocks. The new top layer already has layout B Step 3: Break blocks marked with X and - in the next layer (A). Blocks marked with + in the new top layer (layout B) are relieved and drop. The top layer (layout B) is now gone and dropped 40% of all blocks. The new top layer already has layout A Repeat steps 2 and 3 to consistently drop 40 % of all blocks in the layers. You can expand this scheme indefinitely in the horizontal direction Bring ladders just in case you find yourself stuck

That scheme is definitely the optimal in the limit with 50% full blocks. I was cooking up a complicated tileable scheme (with a 10x10x2 repeating unit) but it only gives 40% full blocks in the limit. Your scheme is better than mine if n is at least 11, so if your middle part is at least 9x9.