Innertube bands

It’s not the first time I’ve asked a bizarre question on here, but really could use some advice.
I have 4 British m76 lightweight para helmets ( one of which is a nice early smooth version with the wight/cream leather ) and I would really like to finish them off with an inner tube band.
The problem being I have absolutely no idea of the size inner tube needed for the ‘contract’.
I’ve had a swift look on line that has left me hopelessly baffled!
Any assistance would be greatly appreciated from the more technically minded folks out there!

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