You’ve got it right. TPI, Tons per Inch Submersion. (or TPC).- There would be a table in the Stability and trim book and posted somewhere in the house. Depends on draft but C/M and capt likely can estimate without the table.
Maybe it was economics, capital costs vs operation cost but in practice it did seem like the steam ships were faster then the diesel ships that replaced them, at least for a while. The older Tote ships running to Alaska were faster then the newer D7s using low-speed diesels on that same run. Of course the steam ships were using a lot more fuel.