[MENTION=136384]unterhausen[/MENTION] my guess for your square hole would be to size it approximately to 3.97 mm, that is a bit less than 1/2 of Shapeways stated printing accuracy. Their formula is 0.15 mm + 0.15% of the longest axis. Anecdotally when dealing with rather small parts, like the ones I have designed, the accuracy appears greater but does have a tendency to run factionally smaller than true-to-life dimensions. As an example, my Garmin mount was modeled to be 31.20 mm in it's outside diameter and the finished print, regardless of print orientation, on average measures 31.15 mm. Another example, an M3 nut requires a polygon with an outside circumference of 5.5 mm, I typically model a recess for an M3 nut at 5.55 mm so that it does not require force to insert.

Please note that tolerances of Professional Plastic are a bit looser than Versatile Plastic, but also appear to run fractionally smaller than modeled and tighter than the specification on small models..