This code tests the logic used to come up with the stage size and content scale factor on typical devices.

TestDeviceScaleFactors.as

package
{
    import flash.display.Sprite;

    public class TestDeviceScaleFactors extends Sprite
    {
        public function TestDeviceScaleFactors()
        {
            var testSettings:Array = [
                { name: "iPhone 3GS", dpi: 163, resX: 320, resY: 480 },
                { name: "iPhone 4", dpi: 326, resX: 640, resY: 960 },
                { name: "iPhone 5", dpi: 326, resX: 640, resY: 1136 },
                { name: "iPhone 6", dpi: 326, resX: 750, resY: 1334 },
                { name: "iPhone 6+", dpi: 401, resX: 1242, resY: 2208 },
                { name: "Galaxy S1", dpi: 233, resX: 480, resY: 800 },
                { name: "Galaxy S3", dpi: 306, resX: 720, resY: 1280 },
                { name: "Galaxy S5", dpi: 432, resX: 1080, resY: 1920 },
                { name: "Galaxy S6", dpi: 577, resX: 1440, resY: 2560 },
                { name: "Nexus 1", dpi: 252, resX: 480, resY: 800 },
                { name: "Nexus 4", dpi: 318, resX: 768, resY: 1280 },
                { name: "Nexus 5", dpi: 445, resX: 1080, resY: 1920 },
                { name: "Nexus 6", dpi: 493, resX: 1440, resY: 2560 },
                { name: "ZTE A910", dpi: 267, resX: 720, resY: 1280 },
                { name: "Xperia M5", dpi: 441, resX: 1080, resY: 1920 },
                { name: "iPad", dpi: 132, resX: 768, resY: 1024 },
                { name: "iPad Retina", dpi: 264, resX: 1536, resY: 2048 },
                { name: "iPad Mini", dpi: 163, resX: 768, resY: 1024 },
                { name: "iPad Mini Retina", dpi: 326, resX: 1536, resY: 2048 },
                { name: "Nexus 7 (2012)", dpi: 216, resX: 800, resY: 1280 },
                { name: "Nexus 7 (2013)", dpi: 323, resX: 1200, resY: 1920 }
            ];

            for each (var settings:Object in testSettings)
                testDensityToScale(settings.name, settings.dpi, settings.resX, settings.resY);
        }

        private function testDensityToScale(name:String, dpi:int, resolutionX:int, resolutionY:int):void
        {
            var iPad:Boolean = name.indexOf("iPad") != -1;
            var baseDPI:Number = iPad ? 130 : 160;
            var exactScale:Number = dpi / baseDPI;
            var scale:Number = 1.0;

            if (exactScale < 0.875) scale = 0.75;
            else if (exactScale < 1.25) scale = 1.0;
            else if (exactScale < 1.75) scale = 1.5;
            else scale = Math.round(exactScale);

            var stageWidth:int  = Math.floor(resolutionX / scale);
            var stageHeight:int = Math.floor(resolutionY / scale);

            var viewPortWidth:int  = stageWidth * scale;
            var viewPortHeight:int = stageHeight * scale;

            trace(name + " => " + resolutionX + "x" + resolutionY + "@" + dpi + "dpi");
            trace("  -> scale factor: " + scale);
            trace("  -> stage size: " + stageWidth + "x" + stageHeight);
            trace("  -> viewPort size: " + viewPortWidth + "x" + viewPortHeight);
        }
    }
}

Here's the result of that code:

iPhone 3GS => 320x480@163dpi
  -> scale factor: 1
  -> stage size: 320x480
  -> viewPort size: 320x480
iPhone 4 => 640x960@326dpi
  -> scale factor: 2
  -> stage size: 320x480
  -> viewPort size: 640x960
iPhone 5 => 640x1136@326dpi
  -> scale factor: 2
  -> stage size: 320x568
  -> viewPort size: 640x1136
iPhone 6 => 750x1334@326dpi
  -> scale factor: 2
  -> stage size: 375x667
  -> viewPort size: 750x1334
iPhone 6+ => 1242x2208@401dpi
  -> scale factor: 3
  -> stage size: 414x736
  -> viewPort size: 1242x2208
Galaxy S1 => 480x800@233dpi
  -> scale factor: 1.5
  -> stage size: 320x533
  -> viewPort size: 480x799
Galaxy S3 => 720x1280@306dpi
  -> scale factor: 2
  -> stage size: 360x640
  -> viewPort size: 720x1280
Galaxy S5 => 1080x1920@432dpi
  -> scale factor: 3
  -> stage size: 360x640
  -> viewPort size: 1080x1920
Galaxy S6 => 1440x2560@577dpi
  -> scale factor: 4
  -> stage size: 360x640
  -> viewPort size: 1440x2560
Nexus 1 => 480x800@252dpi
  -> scale factor: 1.5
  -> stage size: 320x533
  -> viewPort size: 480x799
Nexus 4 => 768x1280@318dpi
  -> scale factor: 2
  -> stage size: 384x640
  -> viewPort size: 768x1280
Nexus 5 => 1080x1920@445dpi
  -> scale factor: 3
  -> stage size: 360x640
  -> viewPort size: 1080x1920
Nexus 6 => 1440x2560@493dpi
  -> scale factor: 3
  -> stage size: 480x853
  -> viewPort size: 1440x2559
ZTE A910 => 720x1280@267dpi
  -> scale factor: 1.5
  -> stage size: 480x853
  -> viewPort size: 720x1279
Xperia M5 => 1080x1920@441dpi
  -> scale factor: 3
  -> stage size: 360x640
  -> viewPort size: 1080x1920
iPad => 768x1024@132dpi
  -> scale factor: 1
  -> stage size: 768x1024
  -> viewPort size: 768x1024
iPad Retina => 1536x2048@264dpi
  -> scale factor: 2
  -> stage size: 768x1024
  -> viewPort size: 1536x2048
iPad Mini => 768x1024@163dpi
  -> scale factor: 1
  -> stage size: 768x1024
  -> viewPort size: 768x1024
iPad Mini Retina => 1536x2048@326dpi
  -> scale factor: 2
  -> stage size: 768x1024
  -> viewPort size: 1536x2048
Nexus 7 (2012) => 800x1280@216dpi
  -> scale factor: 1.5
  -> stage size: 533x853
  -> viewPort size: 799x1279
Nexus 7 (2013) => 1200x1920@323dpi
  -> scale factor: 2
  -> stage size: 600x960
  -> viewPort size: 1200x1920