to get an accurate measurement as close as possible to the torque the nuts are done up to dram a line accross the nuts' centre and extend it to either side of the surface the nut is done up on - then undo the nut and redo it up measuring the torque with a torque wrench until the lines you marked previously line up again then read the torque setting and you should be pretty close to what it was before it was undone
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