My trans temp gauge never seems to move so I bought a Scanguage. Now I have digital trans temp, boost press and a couple of dozen other engine and milage parameters. They cost about $170 and plug into the OBDII port.
I do not know how hot is too hot.
2004 F350 PSD FX4 SB CC, Banks Cold Air Intake
18,000 Superglide
2003 37SP Forest River Sierra Toyhauler
2008 Polaris RZR