Measure the head
Wrap a tape just above the eyebrows and around the widest part of the back of the head. Add padding thickness and, for rigid prints, about 30 mm clearance.
Test the neck opening
The tightest point is usually the neck hole, not the dome. Print the bottom ring first with the fit-test section tool.
Watch the visor line
Check eye level against the visor on the test ring before printing the full shell.
Next: more armor guides.