True, it is impossible to keep it between 3.0-3.5 on a consistent basis. That is my target range that was set by my surgeon. I think all that means is that the surgeon wants to keep my range on the high end esp. with my condition (Tricuspid). Realistically, 2.5 -4.0 is what me and my Dr. consider normal without changing the dosage. I've been on Coumadin for a year now and I keep records of all my INR readings in a spreadsheet. My range has not fallen below 2.5 but and it reached 5.0 once. My yearly average was 3.49! Pretty damn close! I know that the overall average doesn't count but hey it sure shows consistenty.
There is a slight learning curve in the beginning but once you get a feel for it, you will be able to self dose and almost intuitively know what your INR is. I can always predict my INR level within .5 depending on diet and exercise and alcohol that week.