Next time you tell her it's 12mm. Tell her with some confidence. Measure it first by creating this handy little jig. Just drop the bad boy in the right spot and now you know for sure.
Just take a ruler make a photo copy or two to cut out. A short piece of corner molding. I put an indent in it where you put the head to measure it from. I had some extra time LOL so I also drilled a couple holes and put a screw in the size I was sure or then added the photocopy of the ruler to it. All the measurements start the bottom of the head. She will feel so much better knowing that when she asks for a 12mm cap head. That is exactly what you are going to be giving her.
Warning side effects range from hot dog finger to severe vomiting. Use at your own risk. Vitalis.....