50mm Calibration Slab for checking inside dimensions and thin walls

I printed out a 1 inch calibration cube and got my Printrbot LC calibrated really accurately. Or at least I though so until I printed something with inside features (holes and other openings) which turned out to be way too small. I made this 'cal slab' so I could check a number of sizing parameters after tweaking my printer and Slic3r configurations. It's only 6mm tall because I have had not issues with Z calibration. I also added a number of thin walls to try and get an idea what my actual