Where do you take the temperature measurement on a dry type transformer.?
You should take your temperature readings from the windings as this is the component you are protecting by measuring the temperature. Over time, the e.m.f of the windings will cause small movements within resulting in work hardening of the copper. This will cause higher resistance paths and hot joints. Also, the surrounding environment can cause overheating. If the ambient temp is excessively high or no air flow passes over the transformer, this may assist in overheating.
You can take the temperature reading either way, just know how to translate that temperature reading into an awareness of when and how much cooling you need. If you know that the temp in the core is WAY hotter than the wiring, and you take your temperature reading from the wiring, just know that you need to compensate your awareness of the heat by applying cooling at a lower temperature, relative to your core temp. If you take the temp from the core, you can get a better idea as to when and at what rate the heat is accumulating, because that is your source of the heat. That would be better in my opinion.