Short answer : about 34 f for me.
(you can stop reading now....)
So far we have from 7f to 40f , that should be enough to make anyone realise that there is no correct nor an easy answer to that question.
Maybe there is a better way.
Thermarest rates (using the same standard...) the Prolite Plus at R 3.8 , the Neo Air at 2.5 and the Z Lite at 2.2.
A blue mat is about 1.5.
( if you combine a blue mat and a Neo Lite (1.5+2.5) you get about the same thermal resistance as the Pro Lite Plus (3.8))
So if you like you can think that a ProLite will take you down (at least) to a temperature twice as cold as a blue mat.
The problem here is that there is no good answer to , say, : what is twice as cold as 32f?
So how can you work this out ?
Well you could ask : having used the two mats, at what temp you are still comfortable with a Pro Lite 3 and the same for a Neo ?
The person that matches your temp for the ProLite may be close to what you expect for the Neo....
Averages tend to work in statistics, but in reality no one has 1.5 children.
Franco