Really need a refractometer to make it easy, but I've found that if I let the beer settle in the container and spin the hydrometer a bit when putting it gently in, it reads pretty faithfully for about 10 seconds before the bubbles start to elevate the hydrometer.
Remember to measure the temp and correct for the calibrated temp of the hydrometer. Of course if you're not sure it's right when you do this method, RB's decarbing works better, just takes longer
Next time south, I'll post a note so we can get everyone a refractometer at half the price I've found them up north at.
JQ