I couldn't get used to a helmet mounted mirror. I have been using the Hafny Bar End mirror (62mm) on all my bikes for the last few years.

Large enough to easily see, easy to mount and adjustable.

Amazon.com : Hafny Bar End Bike Mirror, Stainless Steel Lens Bicycle Mirror, Safe Adjustable Rearview Mirror, Cycle Mirror, E-bike Mirror, HF-MR083 (Black 62mm) : Sports & Outdoors