How do I achieve required torque when using an impact driver?
"How do I achieve the same torque repeatedly for multiple nuts when using an impact driver?"

Short of manually using a torque wrench like woodchips recommends, you don't.  The point of an impact driver is not to apply a consistent torque.  It is to apply more torque than can be obtained from a traditional driver.  One of the more common uses is to loosen seized up, or over-tightened bolts/screws.  I also use mine quite a bit when driving screws into wood when the final torque doesn't matter (e.g.- I just need the screw head to be slightly below the surface of the wood..  rough work).

In other words, if your need is consistent torque, an impact driver is not the tool for the job.  A traditional clutched driver is going to be more appropriate.