The June 2007 issue of Audio Xpress magazine has a 4 page article by Daisuke Koya:
"Radio Shack Sound Level Meter Characteristics"
Although I don't agree with his protocol, it's still interesting reading.

Interesting thread (http://www.hometheatershack.com/forums/bfd-rew-forum/361-radio-shack-spl-meter-correction-values-all-new-cal-files-published.html) on hometheatershack forum, too.