Save time, effort, and get the best fit!
TLDR: When you buy a good suit, you have a tailor measure you for a perfect fit. You can spend all day trying off the rack, but save yourself the time and get the most accurate fit by having the guy with the measuring tape figure out what you need.
When I started riding, I spent a long time trying to pick out a helmet. With dozens of helmets in front of me in the shop, picking one up, taking it off, moving to the next...I may as well just roll dice to make my choice. I found an XXL helmet that fit me, I thought, perfectly.
On reflection, it didn't. I didn't understand in the beginning that manufacturers make helmets with some concept of head shape. You want the most perfect fit, but how do you know what your true requirements are. Even more important...how do you know what helmets would serve your noggin best?
I purchased a lid picker kit, emailed my data in and the same day got an email back; they asked me to re-measure, because they thought I may have been outside the normal range of heads. With the help of a friend, I found out one of the dimensions I had sent was wrong.
So. Kudos to the folks at lid picker for responsiveness, and a recommendation (as they said) to have a friend help you measure.
With the new data I was quickly emailed back a report that listed what I assume is every major production helmet you can buy, and they compare their fit/shape with my own head data to determine how each helmet would fit; some would be in range, some near an outer limit, some spot on...and some wouldn't fit no matter what. It took into account size as well.
With this report, it was easy for me to determine the three helmets that best fit my head and order them. I have to say, the two I've already gotten fit perfectly close. It was a surprise to me, since my first helmet was an XXL, that I ended up in size L in most designs that fit me. If I had tried them on in stores I would never have even got them over my ears and realized that once mounted, they were perfectly fit.
I for one am pleased at the turn of events - and kudos to Aaron for the feedback that sent me back to the measuring tape.