Can FME calculate TVDSS using measured depth?

That depends on the calculation to be done.

Usually the ExpressionEvaluator is the transformer to use for that, but since it is embedded in other transformers such as the AttributeCreator it is possible to make use of the math functions elsewhere.

@shimah you might find this presentation from the FME World Tour interesting: Well Directional Survey Processing by Jerrod Stutzman of Devon energy