.8 mm too big!

Hey all I have finally got my prusa i3 going and printing some good stuff. However everything is 0.8mm to big in the x and y directions for example If I print a 20mm test cube its actually 20.8mm in the x and y directions. How do I go about making my prints dimensionally perfect? I have checked that the x and y axis both move 100mm when commanded to in pronterface so I know they are calibrated correctly.

At the moment I am just adjusting the scale in slic3r to get the dimensions correct but there must be a proper way of doing it?

Have you checked internal dimensions of holes. What I have found is that most parts come out about 0.005 inches oversized external and undersized internal. I believe this is due to the filament squishing out when extruded. It is the natural expansion that allows one strand to bond to the next. The outside strands do not have the adjacent strand to constrain it so its over sized. It would be nice if the slicer program had a compensation parameter to control this. What I do is for interfacing parts is offset the edges inward that 0.005 inches and the parts fit perfectly.