Isn't the transmission cooler shared with the engine coolant? And anyway the transmission case will get more or less in thermal equilibrium with the engine case, since they are bolted together.Steady 65mph on the highway = ambient temp + 75/80F (depends on absolute ambient and humidity).
Sitting in traffic = ambient + 95F/100F
In this case the transmission fluid temps should be monitored in relation to coolant temps.
I am looking at keeping them equal or at most +10F for transmission (in heavy usage like stop and go traffic in 90F or higher ambient).
On highway usage, I am at -10..20F because I have added an aux transmission cooler.