When you put it in, you don't want the tip to bottom out and touch the casting on the other side

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Just for knowledge, I thought it was because coolant flow starts at #1 and flows to #6 thus the coolant is almost as hot as it can get by #6 not absorbing much more heat.

 

That would be correct. Dodge mitigated the issue to some extent starting around the early common rail era by tapping the heater core out of the head between 5 and 6 to improve flow.

Prior to that, Cummins sold kits that flowed out the rear freeze plug for this purpose. Don't know if they are still available.

Wicked Diesel and Opie had a crack at blingy high-dollar kits using a regulator routing it back to the water pump ostensibly to reduce water jacket pressure at high rpm. Not sure how much bank that deposited but it was a nice bragging point for people with more money than sense.

This phenomenon is common to any inline 6 engine design, gas or diesel.

 

So is it a progressive increase if you do post turbo or is it just flat 200 degrees

 

In cruise, they match back up. Under power is when they split.

 

Oh gotcha. I'm just going to go pre turbo.

 

That's the best place, mainly because you will know for a bit more certain when you bust that 1250° danger zone. Post turbo can work if you can remember to figure in a correction based around boost and you know how much you have to correct for based on the probes location. That 200° is not a straight across the board figure because of location.