SONarchitect calculates the parameters based on models that are dealt with in paragraph 4 of ISO 12354-11.
The DnT parameter is calculated from R', taking into account that T0 is 0.5, as extracted from the formula (7):
SONarchitect calculates R' from the formula (1).
DnT depends on the volume, but not on the actual Reverberation Time of each receiving enclosure.