Gauging a dial caliper for reloading (9mm)

[JONABI]

I've lost my first reply( took too long, I was logged out).
Now I've zeroed my caliper: good. Measured a 9mm cartridge first reading mm=29,15 converted in inches= 1.1476 rounded to 1.148 compared to vernier combined with dial measurment= 1.145. Difference of at least .003 inche. That's why I want to calibrate my caliper because I won't be sure of any measurment until I can gauge it. I understand that I will face many variables while reloading. But at least I should know for sure what's the real reading before I start reloading to expect some kind of accuracy.