+1 Silv! Much more precise.
Here's a great link that I like to use. There are plenty others that aren't branded, but yeah, I'm a Corsair fanboi
Only thing I don't like is they don't have a 560Ti listed, which baffles me given how popular that GPU is....so I plug in the 580 just to be conservative.
Remember, PSUs have an efficiency curve. So you don't want to be pushing the limits of your PSU....less efficient, shorter life, louder fans.