For the record, the easiest and most obvious point.

For rectangular walls, the maths is straightforward:
Inner length = Wall length - wall thickness
Outer length = Wall length + wall thickness
etc

ok

I think you're wrong. If I understand well how the walls are measured, then for a rectangular wall the middle length (or simply "length") is just the same than the outer and the inner lengths. For walls with one or two oblique sides (because of the connection with another wall) then the inner and outer length depends of the thickness, yes, but must be calculated with a bit of trigonometry [;)] rather than simply add or subtract the thickness.

I might be wrong, of course, but I don't think so.