Use a sizing head. Budwin on thingsverse, has two versions. A lg set of heads and an original set with small medium. Now it should be simple for you to make perfectly fitted helmets as long as you know yours or the persons hat size. I also take a frontal glasses measurement. IE if your hat size is 7" 1/8" and your glasses measurement is 6.5" you will be able to make sure the measurement on the helmet match up and it will fit right first time vs multiple failed size prints. Just a thought.