Wow, I'm glad I opened this before heading out to work-- I don't completely get the equation, as I stupidly took the minimal math in high school (very little is required). However, I intuitively get how making a triangle with two radii and the chord is what I was missing and how the solution for the arc follows. When I get a chance I'll try to educate myself on sine and arcsin, and I may be back with questions.

Thanks guys.