What, you mean like these?

The number of control points you want depends on the angle covered by your arc.
0°: 2 points
0°-20°: 3 points
20°-170°: 4 points
170°-200°: 5 points
200°-300°: 6 points
300°-350°: 7 points
None of this is really exact, just a general idea. Similar to waves, the further the control point is from the start/end in order, the further away from the curve it goes. Make sure the first and last control lines are pointed in the direction you want your loop to begin at. The others, just adjust by eye until it's mostly round. Using an approach circle as a guide can help with this.
Like with most shapes, keep one (two if the number of points is even) control point to take out of grid snap so you can line up the endpoint.
Edit: In other news, loop sliders for lazy people:

The number of control points you want depends on the angle covered by your arc.
0°: 2 points
0°-20°: 3 points
20°-170°: 4 points
170°-200°: 5 points
200°-300°: 6 points
300°-350°: 7 points
None of this is really exact, just a general idea. Similar to waves, the further the control point is from the start/end in order, the further away from the curve it goes. Make sure the first and last control lines are pointed in the direction you want your loop to begin at. The others, just adjust by eye until it's mostly round. Using an approach circle as a guide can help with this.
Like with most shapes, keep one (two if the number of points is even) control point to take out of grid snap so you can line up the endpoint.
Edit: In other news, loop sliders for lazy people: