@Lebowski To directly answer your question about adding the central angle, the screenshot shows where you can change the curve input from Arc Length to Delta Angle, which is the same as the central angle.
However, about this legal description:
First, you do not need the central angle to construct the curve because the arc length is already provided.
Second, the phrase “along a nontangential curve” requires additional directional information, such as a tangent bearing, chord bearing, or radial bearing. Without one of those values, the direction of the curve cannot be determined.
It appears that the described boundary is located 60 feet from the referenced centerline. Therefore, you would need to have the “said centerline” already drafted before you could construct the offset curve correctly. Without that centerline or additional curve-direction information, there is not enough information to determine the curve’s orientation.