Hey, here are some dynamo experiments regarding ceiling heights (from a linked file however):
Are your soffits on the same rvt file or on a linked one?
Do you have many different soffit heights?Or do you mean the structural intrados? In large projects with same structural floor thicknesses I create levels in Revit on the underside of the floor itself and use those to place (or move) pipes, so that i immediately have the negative offset you are looking for, but obviously this works only for homogenous projects…