Nice idea. Perhaps you could add a scale slider in addition to opacity. I know that in my case I’m delivered designs at 2x resolution and would have to view them at 50% size in order for it all to match up. Thanks
What I have done currently is I have just set the width of the overlay to 100% of the browser width, so if a front-end dev has his browser width set to simulate a mobile device for example, it would just scale automatically.