Strutt|Environmental|Reflection from Finite Surface calculates the reflection gain from a finite-surface based on the source-reflector geometry, the size of the source and the size of the reflector.
Depending on the geometry of the situation, the strength of an individual reflection will range from 0 dB to 3 dB
Note that this calculation only considers a single reflection path. If multiple reflection paths exist, either apply the calculation separately for each reflection or (if semi-reverberant conditions exist) use the Semi-Reverberant Courtyard calculation
Reference: Miller, L.N. Noise Control for Buildings and Manufacturing Plants Figure 6-8