Post cooler doesn't really have a meaning unless you tap into the return line for the gauge. The pan temp is a mix of hot fluid from the TC, more in fluid coupling less in lockup, but not all of the output from the TC is routed to the cooler. These is pressure valve that limits what goes thru the cooling system. The pan is also a mix of the fluid after is has been cooled and run back thru all the drums, clutch packs, bearings and thrust washers to cool and lube them. It is aggragte reading thta is easy to obtain.
The other easy place is a probe in one of the test ports. That is just reading heat soak in the tran. Test ports have no flow so they never see real fluid temp just heat soak from the whole trans.
Either place will dramatically affect the actual readings depending on load, ambient temp, driving style, etc. Typically they will read much lower than maximum fluid temp right out of the TC.