Seems like a good idea but from my experience, helmet comfort would be hard to predict with just measurements. Maybe this tool gets you into the right ball park but not to home plate.
I think there are too many other factors that would affect helmet choice such as chin strap placement & ease of use, shield motion, eye port shape, ear pocket location, cheek pad thickness etc, etc. I just bought a new helmet after too many hours reading reviews, rider forums, etc. My research led me to 3 finalists but I could decide until I wore the candidates in the store for a long time and did a side by side comparison of features.
FWIW, I chose the Shoei RF1200. The fit is great, the shield is of very substantial construction and opens and closes nice with solid detents. In use, it is quiet with windshield in any position I normally use. Very happy with my choice.
I also shopped the Arai Signet Q. The fit was perfect. What turned me off was the gimmicky brow vent face shield. I'm sure in practice, it works well. But it seemed flimsy compared to the Shoei face shield. That and it cost $125 more than the Shoei. I tried hard to fit into the Shoei GT Air because of it's purported quietness and airflow. But that internal visor stole too much forehead space from the helmet's inside space. It would have been an issue for me after 30 minutes of riding. I also tried on the Neotec which fit better than the GT air, but was not as comfortable as the RF1200 or Signet Q. If it was I would have bought it. The flip up worked nicely and the internal visor was the best of any I've seen. But that Neotec is physically big and heavy. I'm not sure how that would affect comfort and fatigue after a long day in the saddle.
I was going to write a RF1200 review separately but I guess this thread just drew it out.
Regarding MotorcycleGear.com: i've bought alot of stuff, including two previous helmets from them and they're great to deal with. I hope this tool helps them sell more helmets. I wouldn't have worked for me on my most recent purchase but for many it will. I was happy to give my money to a bricks and mortar establishment that provided the helmets for trying on to make the final decision.