API 2000 SEC 3.4.3.2 only says Emergency Venting For Fire Exposure For Double-Wall Refrigerated Storage Tanks ..... becos emergency venting for a double-wall refrigerated storage tank is complex, no calculation method is presented here.  A thorough analysis of the fire relief for a double wall refriegerated storage tank should be conducted.
Regarding using the API-2000 method to determine heat input, I don't see that either API-12F or UL-142 reference using API-2000 as a means of determining venting capacity.  Both API-12F and UL-142 ball valves say to follow the tables included in their respective standards to determine venting requirements versus wetted area, there does not seem to be a heat input calculation method.  I would be cautious about using a heat input method along with a latent heat value to determine venting capacity especially if it would result in a venting capacity less than what the tables would indicate for a given wetted area.  Ultimately, I believe you must comply with the venting requirements as defined by the standards to which the tank is constructed.  Now, since the venting capacity versus wetted area tables in API-12F and UL-142 are based on NFPA-30 and the table in API-2000 is based on NFPA-30, I think it would be reasonable to use the table in API-2000 and get the same result.

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