It will also calculate the area of the sector with that same central angle. The calculator will then determine the length of the arc. See Calculate Field examples.
How to Calculate the Area of a Sector and the Length of an Arc.
If you know this then you can use the formula maths. Use the tangent function if you know the length of the opposite side and the adjacent side. 3-15 of the length of line depending on the systems design for a horizontal lifeline. Then my fourth command In4 tells Mathematica to calculate the value of the integral that gives the arc length numerically as that is the only way.