You should always write content for users and not search engines. Trying to achieve a specific percentage of keyword density could do more harm than good if it messes up the natural flow of your content. Instead you should use the on-page optimization tool to make sure your following best practices. It will of course let you know if you should try and include a few more instances of your targeted keyword. Adding additional content sections to the page can sometimes help you find better places to work in your keywords, try adding testimonials, lists, pull out quotes or excerpts of the main content, etc. There's also data that suggests partial match and related terms can help influence your on page relevancy so be sure you're working in these types of terms as well.

I believe the tools are focused on a minimum number of instances of the phrase and not the %. Keyword density as an important ranking factor is a myth.

Here are two great links that will help shine some light on the subject for you:

http://www.seomoz.org/blog/10-myths-that-scare-seos-but-shouldnt-whiteboard-friday http://www.seomoz.org/beginners-guide-to-seo/myths-and-misconceptions-about-search-engines