Please keep in mind that the "movie accurate" helmet that a number of people now have isn't truly "movie accurate." It more accurately should be called "most movie accurate to date" helmet. In reality that helmet is a custom scuplt. It is dangerous to supliment artistic interpretation in place of the real deal.
Please do not think I am trying to tear the helmet down. I think it is the best damn thing I have seen and far ahead of any of the competition. However, the sculptor was just as prone to error as anyone else who has studied the helmet. Instead of using his dimensions as gospel, it would be highly advisable to do an in depth study and discover what YOU feel like are accurate measurements. Otherwise you end up with a trickle down effect of errors going from one sculpt to the next. Always go back to the source. Always rely on the REAL helmet.