When you are dealing with numbers small enough to use a caliper, and you are there, you need to keep the caliper parallel to the retainer and seat. There isn't much room and you can easily be off .030-.040 with what you are trying to do. Unless you have a special caliper. You'd be better off with a snap gauge and micrometer. As you might be able to tell, I'm not a fan of calipers. On a good day, they are accurate to only a few thou depending on what you are measuring.
You are close enough to coil bind that you need to be more accurate in measuring.