If you have a way to monut securely on a milling machine, you can find dead center by measuring from side to side of the receiver, divide in half and rotate head to center. I have a special fixture that mounts the barrel in V blocks, and the jug is automatically dead center. Like this one!

https://www.brownells.com/gunsmith-t...-prod6638.aspx

Look at the fixture it may give you some ideas!