[tallstack]

I am responding to your calcs with reference to Tbag's diagram. Evan's equations are correct and you do need to divide by 2b (or 2y using your variables)

From TBag's diagram, we do not know the distance from the center of the circle to the chord, y. We know the distance from the chord to the outside of the circle. So in his diagram, y should be replaced with R-y.

If you use TBag's diagram then the correct equation is
R^2 = (r-y)^2 + x^2
or R^2 = (R^2 - 2Ry + y^2) + x^2
or 2Ry = y^2 + x^2
or R = (y^2 + x^2) / 2y

From your numbers,
x=.419, y=.251

Radius = .4752
Angle = 2.1589 rad
Arc Length = 1.0259

You are right that the arc length has to be more than twice the hypotenuse.

I hope that this helps.

Dave