It's not an "estimator" -- it's a "guess-timator." Such things only have limited value. Many examples in other fields -- even financial budget projection and PERT project management models -- break down the problem into smaller and smaller components, each with its share of uncertainty and error. This attenuates cumulative estimation error to some degree, because some things are more accurately measured or estimated, while other things have a wider variation of possibilities.
I would never use a program like that to gauge a PSU selection so close -- or within 20W of the estimate. Like I said, my own requirements -- and then only at peak usage, which is very unlikely -- put me at an extreme requirement of 430W, and I still picked a PSU that offers another 90W of continuous power.
But my point of agreement about the calculator is that it only suggests requirement level to which you'd want to add some margin of safety. Also, I think many of the PSUs -- very good ones -- are more dependable or efficient below their rated continuous capacities.
It just doesn't make sense to buy a 600W PSU if your extreme usage plus safety-margin is below 400W.
