Instead of the percentage of income at retirement offer the option of a monthly value. The percentage makes sense if you're planning on remaining in the same locality and attempting to stay at your same lifestyle level. A monthly value makes more sense if your plan is to move to an area with a significantly different cost of living and/or significantly change your current lifestyle.

The other suggestion would be around social security, investment accounts other retirement income and the age in which these apply. It appears that it starts factoring in at the age you specify retiring at. It would be more accurate to factor that in starting at a specific age. For example if I had stocks and a 401k and social security. These all have different points in which you should take out of them for the best benefit. If I wanted to retire at 55 I need to know how long my stock portfolio could maintain my lifestyle and then at 59 and a half I can begin to factor in my 401k. Then depending on my age and what kind of social security benefit level I want I would factor it in at that time.

This would provide the best visualization to see exactly how things will pan out.