I'm a fan of very precise measurements. Sometimes my digital calipers just aren't big enough and the woodworking clamps come out.*bringing the thread back on track*
HaloGoddess' "How To Scale" and More Tutorial
There are a lot of tutorials about how to scale, and what I would do to avoid confusion is not do any 3D anything. Instead, just measure the 2D height of your head, from the bottom of your chin to the top of your head. That should give you one solid number. Type that number into the "height" of your helmet 2D menu scale settings, and that should give you a decent starting point.
Any modifications you make personally will then be fitted against the helmet you have made. (The VR headset, for example. You'd just build your own attachment for it and stick it to your helmet. How you do that is up to you.)
Good luck! Love to see how this works out!