Pyrometer for C15 single turbo MBN; How hot is too hot?

My truck does not have a pyrometer; I purchased one, and will install it tomorrow. Want to track exhaust temps as I start mods to the engine.

The one I purchased is digital, which has a high temp alarm; the readout will begin flashing when a preset temp is reached.
Just wondering what would be a good number to set it at?

 

After installing pyrometer, took my first trip today, 541 mi. Left Lake Charles, LA drove to New Orleans, loaded (gross wt. = 75,480), returned to Lake Charles. Flat ground except for three high rise bridges which cause my speed to drop from 65 mph to around 35 mph. Hottest temperature reached was 894 degrees F., and that was really gouging on it while bridge climbing. So much for setting the high level alarm at 1100 degrees.

 

Shortly after buying that truck, (last April), the yellow "check engine" light would come on momentarily when climbing a high rise bridge, or sometimes even an overpass. It doesn't stay on long enough to throw a code, and although it has begun to light up more frequently, still momentarily, still no code. That may have something to do with the "no power" ailment, as it is weak even for an MBN motor. After I began working the truck, I first suspected it was derated to a 435, or even less, but according to the 'puter, it's supposed to be 475. Mechanics suspect either turbo sensor, or some atmospheric sensor, and advised that faulty sensor would eventually just quit, then show a code, whereas I could replace it, but the #### thing won't die.
Guess I just need to start replacing parts.