I guess the point is you can make an estimate of the CIT based on the geometry of the plane, and the speed and altitude it's traveling at. And similarly the CIT limit can be estimated from published data about the engine plus knowledge of the limits of the materials used when it was designed.

This is actually an engine limitation. It's to keep the turbine blades from melting. Unfortunately, you can't measure the turbine inlet temperature very well, because of the whole melting thing. So they determine what the limit should be, and then back calculate it to somewhere they actually can measure it (which is the front of the compressor). So, you could exceed the CIT limit, but you would run the chance of damaging the engines. The MiG-25 has famously destroyed it's engines by running to fast.