Pepakura helmet scaling


New Hunter
Hi Guys

am currently trying to build my first helmet for my son, and am getting confused with the scaling options in pepakura. While i know how to do it, the question i have is. If i go off the height of my sons head and put this value in pep, with it use this measurement for the outside or inside of the helmet, if outside how do find out what the inside dimentions will be.

Thanks in advance