Sizes Smizes

So if any of you has tried to make a hat you will realize that it is not as easy as it looks. I have been working on a line of baby hats for my etsy shop... They will be up next week so feel free to take a look. I decided to make my own pattern and I worked one up so fast. Then I gave it to my model... a 3 month old baby boy. It was of course way too small. I did some research online and came up with this chart:

• months.............circumference.............length
• 0-3....................14 inches.......................4.5-5 inches
• 3-6....................14-16 inches.................5-5.5 inches
• 6-12..................16-18 inches..................6-6.5 inches
• 12-24................18-19.5 inches..............6.5-7 inches

Now according to my model the chart is off by 1... her 3 month old has a head circumference of 16 inches... so I made him a 6-12 month hat and it fit great.
Other than that the hats look good, and if I follow the chart the hat comes out correct... with the length substantial for the circumference.

Anybody out there have the same problem I am having? I could use some help figuring these hat sizes out. Do you know of a site that gives correct sizes?