Need help with helmet sizing


New Hunter
Hello all, I've decided to (finally) take the plunge and try my hand at making a Boba Fett costume. I've been looking at a lot of templates (namely wizardofflight's) and sizes of different helmets for sale, but I don't have access to a printer and will need to hand-copy stencils by eye. I'm not expecting it to be perfect because of this, and I'm not entirely sure how large the helmet should be compared to my head, like how much space should be left inside, how low it should sit, circumference, etc. as while I have experience costume making I've never made a full helmet before. This might be a stupid question, but I'd love any help as it's definitely harder trying to get everything measured and sketched out fully by hand onto cardboard without any printed stencils. Thanks in advance!