Just degas your sample before measuring the SG. Shake it up, let it sit at room temp for a few hours, and then measure away. This ensures that you don't get any bubbles forming on your hydrometer that could through the measurement off.
an easy way to get the co2 out of your test sample is to put it into a glass and just stir it with a spoon for a couple of minutes, then pour it into your graduated cylinder to measure. job done