Well... Zen is right. The cap is indeed 80% (667 AR) and it might be important to remember. Many people choose not to wear helmet, so they won't get the hidden +25 - their "armor cap" will therefore be 592.
I don't see why anyone would decide not to wear boots or gauntlets, let alone armor, but if they do, they need to aim for slightly higher AR to get maximum damage reduction.
You are preaching to the quire, Jr. No body is saying that he/she is wrong. As far as I'm concerned you are comparing apples to oranges. The complete formula is displayed on the UESP. Splitting hairs over it does not change anything. If you want to run with a 600 + AR and feel good about, knock yourself out. I think the OP is well covered. Don't you?
I personally don't get any where near that high of an AR anymore. In fact I quit putting any perks in Heavy or Lt. By the the time you get to high levels, your health is / should be high enough to cover it.