A better rule is to make sure it reads well. Don't focus on ratios - that's a huge mistake.
That said, I do recommend that you:
- Make sure you use the phrase that folks search for. If you sell 'running shoes', don't write about 'athletic footwear'.
- Make sure your writing is structure and styled so that your key phrase fits in naturally: When you write, you shouldn't have to force the key phrase into the page. It should end up in headings, captions, etc. because it makes sense. If that's not the case, double-check that you're writing the right thing.
- Check the grade level of your writing. I use http://www.readability-score.com/ most of the time. Grade level can impact bounce rate, which can impact rankings.
Hope this helps