Well i am making this: http://puu.sh/9lKzJ/6c39b12f5e.png
As you can see the circle is quiet good as pollys go,
I did change my save in Solidworks so it is more precise,
But when i do 12 in solidworks, also other prints from thingiverse, Its always aprox 0.2-0.6mm smaller,

As you probably would see is that its quiet hard to get this in a vice to get enough grip to not get it damaged and get it drilled,

I had a few trys so far ^^
Thats why i would like to get it more to the 12 so i wouldnt need alot of grip on the product as it wouldnt take much from it.