When a blood pressure cuff is used as a tourniquet how should the pressure be adjusted?

Run the pressure up until the bleeding is controlled or there's no detectable pulse in the extremity distal to the cuff. -- no more. The exact number depends on the location of the wound you're trying to control and -- of course -- your patient. Note that tournuquettes are the last line of defense in controlling bleeding, and are dangerous to use in that they increase chances of sepsis and may sacrifice the limb. With arms, I think it's almost always possible to control bleeding with direct pressure.