How to calculate arc length I forgot my secondary school maths, so I need to ask to confirm.

Arc Length = Radius*(Angle In Radian)

Is it correct?
Yes. This works because $C = 2\pi R$ and, coincidentally, there are $2\pi$ radians in one full rotation. Clearly, a fraction of a full rotation produces a fraction of circumference.

To be fair, your equation is in fact the definition of radian.
It's approximately correct only because you assume that the arc is the base of a triangle... It only works where you can say that sin(θ) is approximately θ. For small angles only.
General solution using $sin^{-1}$ is also an approximation tho because of the curvature.
If it was me, I would calculate the perimeter with the given Radius and calculate the ratio corresponding to the ratio $\frac{360}{\theta}$.