This generates perfect circular motion centered around the original position of the layer. I recommend that you map the radius and

cycle inputs to Expression Control sliders, and the phase input to an Expression Control angle.

Apply this expression to the position of the layer.

radius = 75; // the radius of the circle

cycle = 1; // number of seconds to complete a circle; higher value = slower

if(cycle ==0){cycle = 0.001;} //avoids a “divide by zero” error

phase = 27; // initial angle in degrees from bottom

reverse = 1; // 1 for ccw, -1 for cw

x = Math.sin( reverse * degrees_to_radians(time * 360 / cycle + phase));

y = Math.cos(degrees_to_radians(time * 360 / cycle + phase));

add(mul(radius, [x, y]), position)

My Work:

http://www.morecompletefx.com