I recommend you purchase and review ASTM D2240, Standard Test Method for Rubber Property—Durometer Hardness. Section 7 explains the methodology for calibrating rubber durometers of all types. You can buy sets of Rubber Test Blocks, but I recommend using them only for quick checks because the Type A durometer leaves indentations in the blocks that accumulate on the surface and effect subsequent readings near the same spot on the block. I recommend you send durometers to an expert source that can calibrate the durometer to the requirements of ASTM D2240. Even at that, there are so many variables associated with durometer testing rubber that it is an estimate, at best.