I don't really understand your problem, but this is what I know, maybe it will help.
The arc length is directly proportional to the angle subtended at the center by the arc.
i.e. Arc length/Circumference=angle subtended at the center/360
or Arc length=angle subtended at the center/360 times circumference
for any circle with radius r, if the angle subtended at the center by an arc is x, then the arc length is
Arc Length=x/360 times 2(pi)r