I think you have a very sensible plan. It doesn't matter what gearing other people use, but what you find comfortable and efficient. One thing I would suggest is to not jump from an 18 to a 16, but to first go to a 17 for a while before going to the 16. One way to assess your progress is to get a heart monitor, and watch your heartrate. Over time as you gain fitness, your heartrate at your set speed will drop. Best results will be obtained with your sustained heartrate between 65% and 85% of your maximum.