Puck.js magnetometer as a compass ?

this is the simplest code that I am using:

var Vec3 = require("Vec3");
var vmin = new Vec3(Puck.mag());
var vmax = vmin;
function onMag(xyz) {
  vmin = vmin.min(xyz);
 vmax = vmax.max(xyz);
 var c = vmin.add(vmax).mul(0.5).sub(xyz); 
 var a = (Math.atan2(c.x,c.y)*180/Math.PI);
if(a <0) a+=360;
  
    console.log( vmin);
    console.log( vmax);
   console.log( c);
    console.log( a.toFixed(0) );
}
Puck.on('mag', onMag);
Puck.magOn(10);

I get consistent 0 to 360 degree measurements, but with 0 to 270 actually between 0 and 350 and 270 to 360 occuring in the last 10 degrees.