Google and other search engines are caring less about keyword density these days. My rule of thumb is to mention the keyword that you are trying to target only once and the rest of the article or content that you creating should flow well.
One way to "persuade" the search engines that your content is actually relevant to the keywords that you are trying to target is to use collocation keywords naturally rather than having a rigid keyword density % as a guideline.
And if you still want to go down the path of basing your content on keyword density, then my way of doing it WAS to simply use the Find function in MS Word or Google Docs, take the number of results and divide by the total number of words in my article. :lol: