I use the seo book free plugin for firefox that gives me a density of all words on the page. I keep mine between 1-4% I have tested different percentages and this seems to do the best for me and my website. Each niche can be different so it is best to test it out for yourself. I don't keyword stuff and always make it readable for humans.

I stay around 2-3%, making some of them bold, underlined, and italics.

read this: Keyword Targeting: How to Employ Multiple Keywords for SEO & Conversions | SEOmoz

A specific percentage doesn't matter.

What matters is on-page related keywords, just don't let the related keywords overpower your main keyword (on-page).

Heck, I have some main keywords around 25% that rank good in the SERPs.

Use your keyword in the first 2 sentences, somewhere in the middle of your post and in your last sentence. And bold it once.

But you must also have your keyword in the title of your post, in the Atl text of an image, in h1 or h2 tag and as the anchor text of a link pointing back to the page it's in.

It depends on many sites' that you have to promote and make your keywords natural.