Help with expression - linking to beam points

i have this expression applied to a flare on a beam effect - it moves the flare up and down the beam.

myAngle = (thisComp.layer("Spark Speed Controller").transform.rotation) / 360;

p1 = (thisComp.layer("Atom Build 1").transform.position);

p2 = (thisComp.layer("Atom Build 5").transform.position);

myVal = 0.5*Math.cos(myAngle*2*Math.PI) + 0.5;

linear(myVal,1,0,p2,p1);

Currently i have to manually edit the start and end points (Atom Build 1 and 5), each time i create a new

beam / spark.

what i want to do i change the start and end points to point at the Beam start and end points of that layer

effect("Beam")("Starting Point")

effect("Beam")("Ending Point")

but whatever i do i can't get it to work...

myAngle = (thisComp.layer("Spark Speed Controller").transform.rotation) / 360;

p1 = (thisComp.layer(effect("Beam")("Starting Point")).transform.position);

p2 = (thisComp.layer(effect("Beam")("Ending Point")).transform.position);

myVal = 0.5*Math.cos(myAngle*2*Math.PI) + 0.5;

linear(myVal,1,0,p2,p1);

where am i going wrong?