12 to 17 percent should be good enough I used the navy calculator online but the only trustworthy readings are off a lab test.
Many gyms nowadays will have the electro impedance test which is pretty good but even that is not fully accurate unless used under controlled circumstances. You can Google it and there are a few easy articles out there discussing the pros & cons of various methods. It's good to use a couple & use them over time as a relative measure of progress rather than any final statement of your condition.
You're not fat at all! The problem with the scale is that it is measuring the legs only. It's not able to measure the upper body. I did find a tip at Mark Maslow's blog marathonfitness.de to buy a Calipierzange and measure the body fat at the core and take a photo in boxers every week. Once from the front and one from the side. It's helping me to get a more realistic view about myself without the necessary tests Patrick mentioned already.