How much noise reduction is just 'fine' and equal to that of iso 100?

Sorry if the question was framed in a confusing way, but what I mean to ask is that I clicked some photographs with my Nikon DSLR D5100 in the zoo at ISO 400.In NEF/RAW format. In my adobe camera raw, how much should I slide the luminance noise slider to get a good result, along with preserving the details? How much noise should I reduce? I can't find that sweet spot. I'm confused. How much noise should I reduce in order to preserve the details and at the same time reducing the noise. I want the image to look close to ISO 100 which is almost noiseless. But I just can't find the sweet spot.

I was photographing birds. My settings were manual mode, aperture 5.6, SS 1/500 and ISO 400.

The reason why I'm asking and want to reduce noise precisely is because I would be submitting them to stock photography websites like istock, fotolia etc.etc. And I don't want them to decline it on the basis of noise. The photos are really fine! Really! And I don't want them to suffer because of noise.

If you can tell me the exact industry standard/number to put in the luminace slider bar, then it would be cool! Ofcourse, if any.

There's no industry standard.you'll have to be the judge of how much noise to remove. ISO 400 shouldn't leave you with much noise to begin with, but ideally, reduce noise just enough to preserve the detail of your shot, while removing as much noise as possible. Its a balancing act for sure.

The correct level of noise is very subjective because there are times high ISO means getting the shot you want vs missing it. The subject and the setting you intend to capture it in also play a role in how to use ISO. Some graininess can be considered good.finding the sweet spot depends on what you consider sweet.

Just a tip. Always shoot with ISO:100 because that eliminates the noise. Not completely because youll need a software for that but if you shoot the shot right with iso 100 then you should have the least noise. Shooting ISO 400+ gets grainier and noisier.