[TimT]

Changing the air pressure to change the power output of the wrench is counter to what most standards accept.. usually an impact wrench is calibrated with a fixed length of hose at a fixed pressure on a calibrated tension measuring device.. (Skidmore/Wilhelm etc)

Vary the pressure, change the length of hose and the applied torque changes..

Keep the air pressure fixed, and since it seems that this is a very controlled environment, the length of the supply is fixed also..

Use turn of nut..

You will have to determine initial torque for the fastener, and how each degree of rotation of the bolt increases the tension..

Unless you have calibrated the wrench at each pressure you have no idea what torque is being applied to the bolt..

Can you use ultrasound to measure elongation in that environment?