Mostly depends on how hard the planned ride is. Typically I start putting embro on in the low 60's and don't put knee warmers on until the very low 40's. if it's wet out i'll put embro on in the 70's.
I don't think it should be used for training, or at least not as a substitute for clothing. I use embro in training but mainly because I'm old and have joint issues. I don't know that I have a lower temperature barrier. I use non warming embro in the 50Fs on training rides if it's wet. Above I don't feel like I need anything.
Training I'll wear embro wet or dry. Non-warming if it's wet, warming if it's cold. For racing I'll used warming embro below 50F but much more sparingly if it's wet since you're going to get caked in mud with that stuff on.
Thanks for the info.