I actually want to find the lowest shutter speed for a given subject,so i can stop down to f8 or 9 and use the lowest possible ISO,the trouble is ime not getting long enough at one go due to the poor weather we have had for a few weeks now.
Why don't you shoot at native ISO with the D7200 and do the rest in post. It does not have any shadow improvement when upping the ISO (compared to other models) which makes it a feature only justifiable when shooting JPEG.