Disclaimer: THERE IS NO MAGIC FORMULA
As with most SEO subjects there is a lot of misinformation surrounding the percentage of keywords you should include within your website text. The video below is a good, simple explanation direct from Google, in summary:
- No hard and fast rules when it comes to keyword percentages
- It can vary by site, subject and area
- You should mention your keyword 2 maybe 3 times, after that it is diminishing returns
- Repeat the keyword to much and your in danger of doing more harm than good
- Make sure you have good, relevant copy and add keywords in a natural way
- Ask someone to read your text to double check that it does sound natural