At what point do you need to bring plumeria in? In other words, if the night-time/early morning temperature is in the 50's F, can the plumeria just stay outside? How about the 40's?

I know that different varieties have different cold tolerance, but just for the average plumeria, if I were to say: "OK, the low is going to be 'X', so I need to bring all my plumeria in". What would that (magic) low temperature be that triggers the "big move"?

Keep in mind, I get heavy frosts and severe freezes, some teens. At some point I have to bring all my plumeria in and don't want to prolong that point by covering the plants with something to extend their outside season.

My rule of thumb has less to do with the temperature than with the moisture. If it's a cold but clear night you're good to keep them out in the 40's. If it is wet and cold (like it is often in Texas) the chance of rot and cold damage is much greater.
Here in Southwest Florida all my Plumeria are in the ground (and therefore stay out). We can do that because our winters are dry. In Texas where the temperature rarely goes below freezing they must be brought in because their winters are wet.
In my 14 years growing plumeria here I have only had two winters with cold damage.
I hope this answers your question.

There might be a case for making this thread a sticky if others feel this information is helpful.

I've been wondering much the same thing...At what point(degree) do I say, ok, that's it...time to pull them in for the season? Or time to just 'cover' for the night? Dampness in the winter can be more problematic than cold(temp)? I'd love to hear more!

We are probably even wetter than you in Jacksonville. The fall is typically our dry-season though. Winter can be really wet.

I am going to follow what my plant shows me..if it starts to drop more leaves, in it goes. Overnights temps are dicey at times. Temps may just register with 55F but windchill may be higher when there is constant cold air blowing. Even my jade plants that are more cold tolerant just gets too hurt when windchill comes along too much and gets rained on, what more for a tropical plant like Plumeria.

I will not make the mistake I made with my other succulent, Sansevieria. Truly hates the cold. So same with my plumies, will give them that treatment, in it goes when we start getting way too windy and just in the 55F range at night. Moreso when we get forecasts of rain in late fall, double trouble.