"My example is a shot taken at 1600 ISO, 5.6 at 1/160, and want to get the ISO down."
If you open up the aperture to f/2.8 that is 2 "stops" open from f/5.6. You are probably already aware that with shutter speed a one "stop" change is either half or double the original shutter speed. It is similar with ISO - a one stop change is either half or double the original ISO. So, for your example, if you open up to f/2.8 you should be able to drop the ISO to 400 (1600 divided by 2 is 800 for the first stop; then 800 divided by 2 equals ISO 400 for the second stop -- or, mathematically, since you know you are dividing by 2 twice you can just divide by 4 (2 squared) to go directly from ISO 1600 to ISO 400).