Aiee! Math and law make W.K.Shuridys head hurt.
I stole the equation from the link to Arnie's above. I did it manually and it seems to work, though math was never my strong point. I'd think if the calculator was faulty someone on Arnie's would've pointed it out by now.
I think the Joule/velocity consensus is the projectile must meet 500 fps AND 5.7J to be classified as registerable firearm.