Definition of the operating temperature
An engine operating temperature is a temperature at which the parts like piston sleeve are expanded to their optimal working grade. This temperature is an ideal setting. In real conditions, the engine's operating termperature is affected by factors such the manufacturer, engine type, cooling head, humidity, fuel, glowplugs, settings of the carburators and outside temperature.
Source: Diverse

What is a normal OT for SSI ?
As far as I could find, no OT is clearly given on the workshp manual or the owner's manual.
Nevertheless here are the temperatures recorded by B. Vogel for a SSII under conditions further specified on the RR Australian forum under the thread "SY operating temperature data" :
After 25 km : 94-99 ° C (202-210 F)
After 56 km : 103-107 ° C (218-224 F).
Source: The Rolls-Royce Forum - Australia

Overheat sensor
On the right hand cylinder head near the back just below the rocker cover is a temp sender unit. If it has a single lucar connector on it, it is the engine overheat sensor. If the engine gets seriously hot, it triggers a warning buzzer behind the dash, If the buzzer sounds, you need to switch off very quickly to avoid serious and expensive damage.
The buzzer should sound when the block temperature reaches 150 degrees C. It goes off again below 140 degrees C. Normal operating temperature is 115 degrees C..
Source: The Rolls Royce Forum - England