Seat post bolts are measured the same way as all screws and bolts, that is length below the head. If the bolt isn't recessed into the ears than the bolt has to be a few mm shorter than the outside width of the ears, so it has room to pinch them closed before running out of thread.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.