I will do my best to explain what I need done, but so far I can not figure a way to accurately get this to measure. I have attached a picture as well.

The idea is I need to measure from the bottom of the headcase (bullet) to the mid shoulder of the bullet where it measure a .400 diameter. At that diameter the length of headspace should be 1.634. But if i use the CAD drawing it is always going to measure to that exact distance every time no matter what it is measuring. So my question is how do i set this up in CMM to find that .400 and find out what the true headspace is (again its a theoretical measurement)