Interesting. After almost 900 miles towing a boat last week and a CGVW of around 11,000 lbs I saw some interesting stuff on my "dual gauges"
To begin with, I suspect that my 2-year-old OEM fan clutch is shot...
The original gauge hits the "center" at about 140 degrees on my freshly added Isspro and remains there until about 215-218 degrees on the isspro, then it will start to "hike up" a bit. at about 220-223 degrees it reads about 3/4's of the way up. I did not try to push it to get 226 on the Isspro to see if the compressor would cut out. I ran warm pulling the long grades and I did not hear the fan pulling so that is why I suspect the clutch. On the down-hill I would drop as low as 170-175 (I run a 160 thermostat).
My observations are consistant with the OEM gauge being "normal" from about 140 to 215...............

One would think a 75 degree swing would merit a but more than that, no?
I was packing a bit of a load tho: