There are limits to how much you can understand users' interaction with your mobile app or site if you just stick to pixels, and never leave your computer screen. You need to get designs on the device, and check if text, buttons and icons are visible, and safely clickable.
The only way to do this on every platform, is measuring with some sort of real life, physical object. Instead of muddling through with other tools, I made a tool specifically for doing this on mobile devices:
http://4ourth.com/wiki/4ourth%20Mobile%20Touch%20Template
