From what im understanding is you are trying to make it a room with a wall on the out side and a wall on the in side,

From what I see you have there, you will need to slect all 6 faces that make up the boxed room and in the modifier list find "shell" then just turn up the outer thick ness.

Alot easyer and alot better than making it % or all that scaleing stuff.

Just try it.