The most accurate reading is JUST before the turbocharger, where the exhaust manifold combines into a single pipe. This is the point where the most heat will be, and more importantly, it is measuring the amount of heat that is impacting the turbocharger.
On the other side of the turbo, the heat is much less - it could be lower by anywhere from 150C to 250C, as the energy from the exhaust is used by the turbine.
If your gauge cannot read 900C, I would put it AFTER the turbo. Before the turbo, you'll get temps up around 750C under full load. Just be aware that after the turbocharger, you may see a 100C variation in what the temperature was.