Need help Scaling helmets for Pep

I've been searching all over the internet on how to scale pepakura helmets to my head and all I've found is how to scale pepakura in general. Usually when I scale the helmet it comes out too small to the point where my ears get caught on the sides or too big.

I found a video where it told me to scale from the inside hole of the helmet but I'm not sure how to do that. Can anyone shed any light on that?