I recently found a site that uses multiple measurements to get the right sized helmet. It is called Lid Picker.
http://www.lidpicker.com/
I bought the measurement kit and it works.
I thought a Bell Helmet was a Bell Helmet. I had a Bell Vortex full face helmet that I wore for several years on my almost daily commute. When it came time to replace it I bought a Bell Qualifier Deluxe. But I was never really happy with it. On long rides I would get a headache and when I took the helmet off would have a big read mark on my forehead.
I ordered the Lid Picker measuring kit and took my measurements, used the online form and submitted my results. The next day I received an 8 page report of helmets and how their sizing related to my head measurements. The report included full face, modular, 3/4, half, and off road helmets. The report does not include every helmet from every manufacture. Lid Picker is part of Motorcyclegear.com so it is helmets that they carry.
The report even included discontinued helmets, like my old Bell Vortex (which was listed as a good fit for me) and the Bell Qualifier Deluxe (which was listed as too round for my head even though the size measurement from the manufacture listed it as a good fit)
The Lid Picker measuring kit is $20, but you get a $15 off coupon at motorcyclegear.com. If you have the measuring kit or the ability to take your own measurements a Lid Picker report is just $5.
Based on the Lid Picker report I received I ordered a Bell RS2 helmet in size large. Really happy with the fit of the new helmet. Wore it around the house and watching TV for a couple hours, it fits very snug, but no discomfort. Went for a 3 1/2 ride on Sunday had no pain or discomfit. Very Happy with the fit.