You just use the angle you have (in radians) and calculate a distance based on that.
Prerequisites: Change a couple of lines above so you can reuse the radians:
var radians = (degrees - 90) * Math.PI / 180; // subtract 90 here
...
ctx.arc(W / 2, H / 2, W / 3, 0 - 90 * Math.PI / 180, radians, false);
Then use textAlign and textBaseline to center the text:
ctx.textAlign = 'center';
ctx.textBaseline = 'middle';
Calculate a position, demo shows text on the inside - for outside (or in the middle of arc) just adjust the dist value:
var dist = W / 3 - 40;
var tx = W * 0.5 + dist * Math.cos(radians);
var ty = H * 0.5 + dist * Math.sin(radians);
ctx.fillText(text, tx, ty);
Hope this helps!