It appears I overstated the decrease in noise by flying higher. I was assuming noise level was reduced by the area rule, but it appears to be less than that. The HAI Fly Neighborly document says:
A doubling of height or distance reduces the level by six to seven dB(A). If the height/distance is increased by a factor of three, the maximum level is decreased by approximately 10 dB(A), which is equivalent to reducing the loudness by half.
If you are interested in reading the HAI Fly Neighborly Booklet, here are a couple links:
Fly Neighborly Program
and a direct link to the PDF:
http://www.rotor.com/portals/12/Fly%202009.pdf
I still encourage you to fly higher whenever possible :-)
Paul