Noise level decays by distance, therefore the following gives a guide as to the correction factors at various distances. The initial reading is assumed to have been taken at 1m.

1m Correct by 0dBA
3m Correct by 9dBA
7m Correct by 16dBA
11m Correct by 20dBA
20m Correct by 26dBA
30m Correct by 29dBA

Regards