A couple years back, Stock Car Racing Magazine had an article on how to check the calibration. If I can find the article, I'll see if I can type it out. I believe it used a "fixed, known weight" one gallon jug of water and a fulcrum to see the percentage it was off at a given torque setting. You just adjusted for the "off percentage" by adding that percentage to your old setting. If you follow me. I'll see if I can find it.
In a while, Chet.