I'm in the process of building a XIV greathelm, patterned loosely on the Von Prankh helm in Vienna.

It is diffucult fo me to decide the helm circumference since it is not clear what kind of padding I could use.

Soeme say suspension liners were used (Osprey book sources), other say there were thickly padded arming caps who would keep the helm tight in place, a must since eye slits are narrow in originals as in my reconstruction, any change in position of the helm would blind the wearer.

I plan to use my standard arming cap and coif of mail , which already makes my headsize big.

For the moment my helm has a circumference roughly 3 cm greater than that of my head plus arming cap and coif of mail.

The first step is to decide what liner to build, and I would be oriented towards an over the coif's arming cap with outer ring, as depicted in an effigy on the Wells Cathedral, but I just need help on this decision.

That cap is not pretty thick though, while this page would suggest some special kind of arming cap did exist.

http://www.medievaltymes.com/courtyard/Coif%20Padding.htm