You might want to get the book Using Insulin by John Walsh, Stephanie. It explains so many of the little details that many doctors either don't know or don't have the time to go over (like ISF)
Your doctor may give you a number or a formula, but really the only way to figure out your ISF is by trial and error since we are all different. When it's moderately high, take one unit of insulin and see how far it brings your blood sugar down in three hours. Do this regularly and write down the results, starting conservative and raising or lowering the number as indicated. Also it may be different for different times of day; mine for example is 30 during the day but 60 at night. (One unit drops me 30 points during the day but 60 points at night).