I like using the body fat calculator at www.zoneperfect.com It's under the resource link.Make sure if you use an online calculator you find one that asks for your measurments and not just your height. Height/weight charts are often very off for people who have higher lean weight. (Probably has something to do with the fact RDA is only 40g of protien and I don't see how that could support any muscle )

The scales aren't too acurate, they can vary depending on how hydrated you are, when you last ate, and so on. Mine often varies as much as 4%, and I know I don't take off or put on that much body fat in one day.

You can also use caliper method form your doctor or your local gym.