Visual Angle.

ang2pix.js

// cal Pix size with given visual angle and screen info.
var scr = {
  xpixels: 1024,
  ypixels: 768,
  width: 360, //mm
  height: 270, //mm
}

var ang2pix = function(screen, angle, distense){
  var targetWidth = angle.h * distense / (180 / 3.14);
  var targetHeight = angle.v * distense / (180 / 3.14);
  
  return {
    xpixels: targetWidth * screen.xpixels / screen.width,
    ypixels: targetHeight * screen.ypixels / screen.height
  }
}

calVisualAngle.js

// @screen
var scr = {
  xpixels: 1024,
  ypixels: 768,
  width: 360, //mm
  height: 270, //mm
}
// @target
/*
{
  xpixels: 250,
  ypixels: 250
}
*/
// @distense
// 120 cm = 1200 mm

var calVisualAngle = function(screen, target, distense){
	var targetWidth = screen.width * target.xpixels / screen.xpixels;
	var targetHeight = screen.height * target.ypixels / screen.ypixels;
	
	return {
	  h: targetWidth / distense * 180 / 3.14,
	  v: targetHeight / distense * 180 / 3.14
	}
}