target edge of null object rather than centre
You could do it the nerdy mathematical way by finding the radius of the circle r, then using trigonometry to target a point on the line between the two centers—the center of the start null and the centre of the circle—that is r pixels back from the centre of the circle. You do that by finding the vector of the line between the two layers, then finding the angle that line makes, and multiplying the original vector, minus the radius by the cosine and sin of the angle to come up with the vector from the original null to the edge of the circle.

This expression would do that (probably, I haven't tested it):

var r = &lt;the radius of the circle&gt;;
var overallVector = thisComp.layer("circle layer").transform.position - thisComp.layer("start null").transform.position;
var beamAngle = Math.atan2(overallVector[1], overallVector[0]);
thisComp.layer("start null").transform.position + [Math.cos(beamAngle)*(length(overallVector) - r), Math.sin(beamAngle)*(length(overallVector) - r)];


Or you could do it the quick and easy way, by making a circle solid the same size as the target circle, parenting it to the circle so that it is lined up with it, putting it directly above the beam layer, and then turning on alpha inverted track matte for the beam layer. That would make the beam invisible inside the circle. Boom!