Hmm...that's strange. I just checked mine, and it's there. Oh, just something I caught. You said you measured you head and its 24 inches? That is 24 inches around your head, right? Don't measure that. Measure from the top of your head, to the bottom of your chin. I just blindly did mine and got 11 inches, or 280mm. Added an inch for padding, 12 inches = 305mm.
Try this way, and try and follow the tutorial.