It looks as if you get along pretty well now.
I have a 0.5mm nozzle. In Slic3r I set the width (perimeter) to 0.6mm.
The hull is 2.2mm thick overall. That means the gap between the shells is: 2.2 minus (2x 0.6) = 1.0mm.
In Cura it is not so easy because it wants a overall shell thickness (which can consist of several perimeters). If it is more then one, you cannot print in spiral vase mode.
How to trick Cura in this case, you can see how "abcona" did it.
He has a 0.5 nozzle and set the flow to 135%.http://www.thingiverse.com/make:180585
In your case you had to set the flow to 150%: 0.4 x 1.5 = 0,6mm.