That is one good reason to change your probe to pre turbo. A post turbo probe isn't really telling you anything useful. There is no magic number to add and say that is what your pre turbo temps are. Thos that have had probes in both locations at the same time report the differences ran from 0 degrees to as much as 5-6 hundred. My truck will quickly cool to 275 usually under a minute under normal driving.
Coking is still a very real problem even with our high tech turbos. I have a friend who is a diesel mechanic at Ford and they see a lot of it.