FEA is becoming common now, so better request the deflections from tank vendor if they happen to use FEA to check nozzle loads. You can request this to be used for your boundary condition. Just take note that FEA software will just give the translational displacements and no rotational displacements. It needs some manual post processing to get the rotational displacements (I think one FEA software have developed a procedure how to get this). Not sure if there are other FEA software which automatically produce rotational displacements.

In my case, since the tank dia. is greater than 120 ft., I use the API 650 formula for getting the rotation of nozzle due to tank bulging.

Normally, I don't model a rigid element to the center of tank. I just model it at nozzle/shell junctions and impose the boundary conditions there (maximum tank settlement from Geotech report at the edge of tank ,displacement and rotations from FEA software or using API 650) and model the nozzle flex at the shell/nozzle junction (some FEA software can generate this nozzle flexibility). Then I submit the generated nozzle loads to tank vendor for verification. I normally do a verification myself (since there is available tool inhouse) to avoid iterations with tank Vendor. API 650 Appendix P can also be used for tanks 120 ft and above.

Just correct me fellow Stressers if any wrong statement I've made.

Cheers!!