I don't know the realy utility of that measurement in the "real world". You are really measuring the 'delta T' (change in temperature of the input and discharge temperatures) to get a clue. It also varies at RPM's, seawater temps, etc.

I think you really want 2 measures that count: The engine heat alarm (internal coolant - a/k/a "fresh" antifreeze or water); and, for a real warning, the raw water flow.

The flow sensor gives you some time if you loose an impeller or get the intake plugged up, etc.; and the engine temparture alarm (and, GUAGE) allows you get a sense that something is happening to the efficiency of your system.

A change in normal running temps is a good sign that something is "happening" before a catastrophic problem. You can get other "signs" which let you know before you blow into an overheating situation. But I won't bore you more with those.