Does RMP*Comp perform some math or modelling in order to arrive at an endpoint distance, or is it simply interpolating from the tables in the EPA's Offsite Consequence Analysis Guidance (OCA Guidance)?

RMP*Comp follows the procedures set out in the OCA Guidance. This means that for some scenarios, the endpoint distance is arrived at via table lookups, for others via calculations. (An example of the latter is the case of pool fires; see Section 12.2 of the Guidance.) However, when table lookups are used, no interpolation is performed (see Section 4.0 of the Guidance).