Well, what I would do is go to the ATS AND DTT website and ask that question.
Not a transmission guru, but as I understand it:
You want to measure the hottest place which is the hot line. You do not want to burn your fluid so it loses its lubricating properties. That is dependent on what you use regular or synthetic fluid. I measure both the hot line and the pan. When the hot line climbs to temps like those the pan temps are about 25* behind and rising as well.
I pull a 13k 5er and get high temps when backing due to low air flow over the air cooler. I have my water temps at 180* before I start backing. The hot line goes to as much as 240* for about 30 seconds. I am going to put an aux cooler (Tru Cool H7B) and an aux fan to keep the pan temps from rising when the hot line temps go up so the fluid going into the transmission (from the pan) is always as cool as I can get it.
I think it will also help in stop and go driving with the 5er. Thanks RustyJC (again

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If I had a long steep slow grade like you have I think I would also add a "microjet (landscaping low volume micro sprayer)" to the aux cooler (so you are not putting water mist into the engine compartment) and use water mist to help in the cooling. A couple of gallons of water and a simple 12 volt water pump from Northern supply could lower those temps significantly.
The first place I would go (since I have a DTT) is to DTT and call Bill K and ask him like you did with Clint from ATS. "I" don't feel comfortable with temps in the 240's as a routiene for any period of time, hence the aux cooler and fan. However Clint at ATS said "ok" and you call Bill K and he says the same, then it is YOUR feelings on it. They are the transmission gurus supreme and should know.
Maybe change the fluid more than once a year? or a larger volume of it? Hence the aftermarket pans.
Bob Weis