[RacersEdge]

I think that drawing is right.

x = 1/2 cord lenght = known
c = cord height = known

In drawing, r (radius) = y + c

x^2 + y^2 = r^2

x^2 + (r-c)^2 = r^2

Now you know r since x,c are known.

sin(angle) = x/r

arc length = (angle/360)*2*pi*r