After seen your third file it's -15 that you must enter inside the Constant Node ! :)

So what will be the more simple : select 2 loop of edges by my system
or the 4 faces of the futur soon Michael ?

The Michael one will surely more polyvalent! :)

And avoid some headaches! :)
And no need to have 2 Selected nodes! :)
Alas seems the Offset "internal" bugged! :(