The temperature differential depends on the amount of work the engine is doing AND the amount of work the turbo is doing. It is not an accurate way to measure. Pre-turbo is the reading you need since that is the closest you can get to the valves and piston crown. These are the parts you want to protect.
Many worry about getting chips in the manifold and damaging the turbo wheel. Starting the engine after installing the sensor will blow the chips out through the fins without damage since they are not moving yet.
If you are still concerned about this you can do what I did. Cut a piece of metal that will just slip between the turbo mounting studs. Loosen all 4 nuts, but don't remove them. Block the turbo up and slide that metal in and drill the manifold. Remove the chips and the metal piece and tighten the mounting nuts.