The closer you can mount the sensor to the turbo, the more accurate the data you get from it is. There is obviously a delay from what happens in the engine to what your sensor sees. The further away you mount the sensor, the bigger that delay...

Normally you can mount them about 3 or 4 inches back from exhaust housing on the downpipe before it sweeps down by the bulkhead.

Just make sure you have it mounted on the top!