Ok thanks. I am studying this subject right now :)
Edit: Oh, I see. setting extrusion width greater than nozzle diameter is to fill the gap between lines by overlapping as explained here under 'spacing paths' title: http://manual.slic3r.org/advanced/flow-math. But why would this affect the "go-to" default? Wouldn't setting distances between two lines to 120% result in the same gap we had earlier? I think the distance between two adjacent lines should be the same as the nozzle width while extrusion width is 120%. Even tough all the distances from left wall and right wall in your design are both multiple of .4 and .5, one side of the print has perfect smooth surface, while the opposite side does not. I don't know what caused this. I have uploaded a photo of it: https://www.dropbox.com/s/z2wpr3uymqri8kh/RpiCaseQuestion.png?dl=0
Don't get me wrong, this is by far the best 3d printable Rpi case I have encountered, but I am really trying to dig into perfection in 3D printing.
By the way, I printed this with UP2 with Zortrax's abs filament
Thanks in advance :)