Key word density depends on the engine that you are targetting. Microsoft generally likes lots of keywords, while a lower percentage is good for google. Generally, I'd just write content to be natural then make sure your keyword is in the title and H1/H2. Then a couple of times in the written content. Its also good to mix and match a little. For example if your keyphrase was "blue dog" It would be good to also write 'the dog had the blues"
Keyword Density is the ratio of a certain key phrase in a page... for me try to focus more on your Content...
For Google, the keyword density doesn't hold much value, you can try for lower densities of 1 or 2%. What matters though is how relevant the keyword is in its context. As far as I know Google is the only search engine that employs latent semantic indexing, which pretty much changes the rules and the worse piece of news is the fact that it is giving it more and more weight. Regards, George
I don't generally worry about it. If you are in a difficult ranking situation and you think keyword density is going to matter: Measure the density of sites ranking on the first page for the se you are targetting. Then match that.
There is no hard figure for keyword density. Just make sure that your content is readable with keywords. Leave the rest to off-page optimization- Inbound Links.