I was out flying my quad @ 500ft in a big public park and a Southwest jet came right over the top of the park during its descent. I tried to descend and fly out of the path just to be safe because I had no clue what height it was at, and the size of the thing from the ground seemed much larger than I usually see flying around. I'm sure it was several thousand feet above me but it freaked me out a little and I'm apprehensive now w/o knowing roughly how low one can safely get w/o crashing before the runway. Does anyone know a formula for calculating the rough estimate for the HGL of a commercial jet during it's descent given the distance from the airport? I ask because I live near 2 major airports and the flight patterns change pretty regularly. In any event, I was flying almost exactly 12 miles from the airport. Appreciate the help.