With this in mind there are three points that you should consider:

1. You do not work for Google or Yahoo! or any of other major search engines (and if you do you're not target audience of this article). You will never know 100% what this "magic number" is.
2. Even if you did know what optimal keyword density was today, would you still know it after next update? Like other aspects of search engine algorithm, optimal keyword densities change. You will be chasing smoke if you try to constantly have optimal density and chances are you will hinder your efforts more than help by constantly changing densities of your site.
3. The optimal keyword density for one search engine is not same as it is for another. Chasing density of one may very well ruin your efforts on another.

So what can you do? Your best bet is to simple place your targeted keyword phrase in your content as often as possible while keeping content easily readable by a live visitor. Your goal here is not to sell to search engines, it is to sell to people. I have seen sites that have gone so overboard in increasing their keyword density that content itself reads horribly. If you are simply aware of phrase that you are targeting while you write your content then chances are you will attain a keyword density somewhere between 3 and 5%. Stay in this range and, provided that other aspects of optimization process are in place, you will rank well across many of search engines.

Also remember when you're looking over your page that when you're reading it targeted phrase may seem to stand out as it's used more than any other phrase on page and may even seem like it's a bit too much. Unless you've obviously overdone it (approached 10% rather than 5% end of spectrum) it's alright for this phrase to stand out. This is phrase that searcher was searching for. When they see it on page it will be a reminder to them what they are looking for an seeing it a few times will reinforce that you can help them find information they need to make right decision.

Final Notes

In an effort to increase keyword densities, unethical webmasters will often use tactics such as hidden text, extremely small font sizes, and other tactics that basically hide text from a live visitor that they are providing to a search engines. Take this advice, write quality content, word it well and pay close attention to your phrasing and you will do well. Use unethical tactics and your website may rank well in short term but once one of your competitors realizes what you're doing you will be reported and your website may very well get penalized. Additionally, if a visitor realizes that you're simply "tricking" search engines they may very well decide that you are not type of company they want to deal with; one that isn't concerned with integrity but rather one that will use any trick to try to get at their money. Is this message you want to send?

Next Week

Next week in part five of our "Ten Steps To an Optimized Website" series we will be covering internal links strategies and best practices. This will cover everything from image links and scripts to inline and basic text links.