I set up my two nodes with the 10% overhead calculated on the real space.
So, my 3TB HDD has 2.7TB of real space and I allocated 2.7*0.9 ~= 2.4TB of space to the node. Similarly, I allocated 1.6TB for the 2TB (1.8TB real) node.
Now that one node is full I see that it has (a lot of) free space, so maybe I interpreted it wrong: is the 10% applied on the theoritical space? So, if I know that my HDD has 2.75TB of real free space I can set up the node to use 2.7TB, if it’s all free?