Hi,
Whether or not your own sizing trouble came from not including the inner surface, I'd say there's valuable information to be gleaned here.
I am in a similar situation, with
my first helmet, based on the WoF templates, having turned out slightly too small.
The frustrating thing is that it's quite easy, during the early stages of a build, to mistakenly believe that it is going to fit.
Then you can spend a lot of time getting the mandibles, the dome, the earpieces right, only to discover that there is not enough room left for both the keyhole section and your head. (I don't believe it is screen accurate to have your nose squashed up against the t-visor.)
I aim to build a second helmet based on the RafalV2 templates, but I don't want to waste time and 2mm cardboard through trial and error.
So I intend to mock up a few helmet bases at different sizes: +3%, +6%, +9%... until I determine what feels right.
Since it's just for sizing, rigidity and aesthetics are not important, so card stock can be used instead of cardboard. Also, lots of details can be left out: the dome surface, the mandibles, the outer sheath. However, you need to include the helmet base curvature, the dome base curvature and above all, space for the keyhole section. These are the elements which influence the available space inside the helmet front to rear, which at least in my case is the critical measurement.
Also, it's not necessary to waste time mocking up the keyhole section, just tape in a matchbox or something to approximate the space required.
I can't advise much about planning space for other components you might intend to include - padding, electronics, ventillation, and so on - as didn't go this far with my first helmet.
I'd be interested to see your progress.
Cheers.