The front brim should not sit more than two fingers' width above your eyebrows. It should not sit any lower than 1/2" above your eyebrows or it could block your vision. If the brim does not sit correctly, try a different helmet that is either deeper or shallower.
7) Adjust the chin strap. When you're sure the helmet stays in place without the harness, adjust the chin strap so that it fits snugly under your chin. It should be able to hold the helmet in place, but not be so tight as to cause discomfort against your throat or make you feel as though you can't swallow or might choke. Some helmets have sliding clips that allow you to adjust the harness for comfort around the ears.
When you find your perfect helmet, you'll discover an added benefit- it will flatter the shape of your face.
Dover Saddlery highly recommends that you adhere to the guidelines provided in the topic, Helmet Storage and Use. Additionally, if your well-fitting helmet becomes loose over time for any reason, it should be replaced with a helmet that fits properly.
Tip: Save your sales receipt and any product and warranty information that accompanies your new helmet. Some helmet manufacturers provide a cost reduction for crash helmet replacements based on the age of the helmet at the time of the incident. Having your original documentation on hand can streamline the process of replacing a crash helmet.