I guess the magic number is going to be different for just about everyone. I know 880k would make me and the DW very uncomfortable at our current ages and lifestyles. However, I do agree with a number of points in the article (e.g. no mortgage or big ticket items to buy (or pay for) and no cc debt going into retirement). I'm not sure the "happiest" are the "master of the middle" but the quote in blue below is pretty much how we live.
"The happiest retirees are masters of the middle. They don't live on the cheap, they're not the most frugal, they don't usually clip coupons, and they don't stay in $49-a-night motels.However, they are not shopping at high-end department stores, not spending $1,000 a day in Europe, either. They have nice houses, not million-dollar homes. They eat in good restaurants, but not in $250-a-meal restaurants. It's really more about not being completely ridiculous with money. Happy retirees aren't necessarily penny wise but they are certainly not pound foolish."