I believe that the seismic weight calculation for each code will be different. But I think methodology will be same. For example, I am using IS 1893. In IS code, specification for seismic weight is Dead Load + Factored Live Load where factor for live load is 0.25 for LL<=3, 0.5 for LL>3 and 0 for roof LL...
The methodology I adopt is, I believe, the easiest to use for STAAD.
Step 1. Create reference loads, definining (a) dead loads (some may prefer to create separate reference loads for self weight, wall loads and floor dead loads), (b) live loads except on roof (may need to split into two if some floors have LL<=3 and some have LL>3) and (c) live load on roof. Additional loads may be required in case of cantilevers projecting from floors, water tanks, etc where higher contribution is specified in the code.
Step 2. In creating the load definition for eq, call the reference loads with corresponding factors
Step 3. Create load cases for DL, LL, LL-Roof using the reference loads factored by 1.
Step 4. Create load combinations as required.
By the above method, loads are defined only once (instead of once in load case and once in seismic weight definition) which reduces chances of errors.