Oops, I should have put a "this is not a joke" disclaimer in the post lol.
Check the
"Custom Leather Belts" section of the website, there's a wall of text there but the video below the photos is helpful for getting a good measurement. Imagine hanging your belt on a hook by it's buckle, start your measurement from where the buckle would make contact with the hook and go to the hole you use most. That's the short version, that's a little different than the normal measurement most sites use but it helps me get an accurate length.