That's where the "Quick and Dirty" Programming kicks in :D
I have just multiplied the "currentDurability" and "maxDurability" with the cubic scale to address the relation between volume and durability. This works fine, as long as all blocks stay the same.
TLDR: Only volume affects scaling and falloff affects all blocks in different ways depending on distance to the root block of the tree.
To come back to your question, the durability falloff depends on the distance to the root block in the tree structure.
I need to take a few more looks, but it seems, that there is a limit at how many blocks receive the falloff. After I placed a few blocks of the same type, material and size in line, the falloff stopped. The lowest durability for an iron block with the size 2X2X2 was 32. This is also the amount of the falloff for each subsequent block. For a 4X4X4 Block, the min. durability was 256.
After this min. durability is reached, all blocks above this (in the tree) increase their durability by the min. durability.
Example 1:
I got 4 blocks of 4X4X4 in a row, the durability is as follows (nearest to farthest): 1024, 768. 512, 256.
Example 2:
I got 9 blocks of 4X4X4 in a row, the durability is as follows (nearest to farthest): 2304, 1792, 1536, 1280, 1024, 768, 512, 256.
Result: The durability has a min. amount. The falloff also affects already placed blocks but instead increases their durability. For the process of scaling only the volume is of interest. At some point, the blocks direct to the root block seem to get even more than the min. durability per block.
Greets,
Mango