For the Calibration cut material I recommend the 7/16" thick MDF/MDO material. It allows for the calipers to easily grip the triangle cut offs and the extra thickness I believe adds additional strength to the points of the cut off triangles so they don't flatten and throw off the readings.
On the adjustable stop make the angle between the arm and the angled stop piece more than 45degrees, 46 or 47 is enough.