I get about 660w at the antec calculator before capacitor aging. I've never been sure where to put that setting. At the bottom it says 20% at least if you plan to keep your psu for a year, which puts me at 770w. It says 25% if you plan to run 24/7, which I have been for the last month, which puts it at 808w.
Is capacitor aging that big of a deal?
20% is like five years plus for quality electrolytics. Besides, capacitor derating on the secondary only causes increased ripple, and the primary caps degrade even slower and have more headroom. Leave it at zero.