I second others' comments here, but I would set the subwoofer level 1-2dB below 75dB (or whatever standard level for other channels) as measured by the RadioShack SPL meter. This is because the "C" weighting curve slightly rolls off at low bass frequencies. In other words, the SPL meter is slightly less sensitive at low bass frequencies, so setting the subwoofer exactly at 75dB will result in a slight over-calibration of the sub.

See this page for more details on weighting curves.