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The `reference_angle` is the position of the prime meridian at the `reference_instant`, which in Principia happens to be J2000, see https://github.com/mockingbirdnest/Principia/blob/ddccea8dc1e2dbdcd8b54fb314bae08b84e1885e/astronomy/sol_gravity_model.proto.txt#L2178-L2189. The prime meridian of Ceres is defined by the Kait crater, see https://sbnarchive.psi.edu/pds3/dawn/grav/DWNCGRS_2_v3_181005/DOCUMENT/CERES_COORD_SYS_180628.HTM#:~:text=The%20pole%20in%20the%20updated,to%20a%20more%20precise%20value. So you need to arrange your terrain map so Kait is at longitude 170.309° at J2000. I know nothing about defining terrain maps, but I would assume that it is mostly a matter of nudging it to the left or to the right until Kait is where it needs to be.