For what it is worth I have breached many ml barrels. I would take the threads per inch and devide 1 by that number. For example 3/4-16 would be 1/16=.0625". Devide that by 360.
Take a good protractor and determine how many degrees you are off, about .00017 per degree. Remove that much from the face and shoulder of the barrel and you will be clocked. I engraved a witness mark when I did this operation.