To properly adjust your seat height, sit on your bike with your hands on the brake hoods and while barefoot, place your heals on the pedals, and slowly spin the pedals backwards (It helps to have a stationary trainer to do this). You should be able to just lock out your knees at the bottom of the pedal stroke. Keep in mind that a seat that is even slightly mis-adjusted can cause knee, leg and Achilles injuries, so even if you are not having pain, I would suggest going to your local bicycle shop and having your bike professionally fitted. A fitting should come free with the purchase of a bike, and should include adjustment of your shoes, seat and handle bar.