If you need to constrain an Arc based on a known arc length you have two ways:

Constrain arc length based on Chord Length (calculated) and Radius (known value)

Or

Constrain arc length based on Angle (calculated) and Radius (known value)

Formula to calculate** Angle=(180/PI)*ArcLength/radius**

Formula to calculate **Chord=2*radius*sin(angle/2)**

The linked file below uses a known Arc Length as an example and has been created in TopSolid'Wood 2017 v6.18