The only reason I don't like your percentage or the 70% either is they don't take into account options to explore. They are based on the assumption that all expenses will stay the same except for the 'cost of working,' such as clothing and commuting. Our mortgage wasn't paid off, but we lived in CA, so were able to move to a lower housing cost state to save money. I realize that not everyone one has that option. The point is to help people look at their income/expenses and engage in some planning that will make sense for them.
Thanks so much for your comment. There are, of course, many variations and I commend you on getting specific about the percentage you will need. The 70% has been a standard average that planners have been using for many years. It's so outdated but no one has revisited it. I was just trying to make it more realistic, but still use an average. Hopefully that will spur folks to, as you say, "look at their income/expenses and engage in some planning that will make sense for them."