I second the no interior supports sentiment. I don't print any helmets or similar things with dome supports. Sure, it looks messy on the inside but that's where foam goes. Saves a ton of print time and material.
Also definitely look at getting Armorsmith. For the cost of what...2 rolls or filament, it saves a ton. If not, look into the "ring method" or sizing.
TLDR: you print out a slice of the helmet/armor at it's thinnest part, use that as a test to see if it'll fit.