^^^ I think/hope I noted that we've tracked our expenses for years. We have a good handle on what we spend now, as worker bees.
My questions were really two-fold - why the resiliency of the 70% rule (by the way, even the SS Admin references it) and are there any things that I'm missing as a non-retired person? Hidden costs, things to be aware of, etc.
@reeshau's post earlier outlines the fundamental reason the 70% (also sometimes the 80%) rule sticks around. It's mostly because most people spend such a high percentage of their income that they need to work until they switch over to SS at age 65 - that is, "traditional" retirement. That is far different than FIRE.
There are a couple of other minor reasons:
1. Financial advisors and SS are biased towards recommending that people work longer. It's much better, from their perspective, to have someone work longer and have too much money and possibly worked too long, than have someone retire too early and run out of money and have to, proverbially speaking, eat cat food.
2. Financial advisors make money by repeated advising sessions and assets under management. The longer you work, the more money they make.
...
As far as your second question, if you've been tracking for years, probably the main categories to think about are:
1. Expensive things that don't come along very often. New cars, big home maintenance projects like roof replacements, kids college, stuff like that. These are things that might not have been captured in your tracking thus far but would be big enough to have an impact on your budget.
2. New expenses that come with being retired. Most of these are, strictly speaking, optional. But this would include the cost of moving if you decide you want to move after retirement, and, more commonly, higher travel expenses if you want to travel.
3. Some people plan for higher end-of-life expenses such as moving into a retirement home and higher end-of-life medical expenses. Some people cover this with LTC insurance or excess savings. If you're on the younger side this could be something that you could perhaps hand-wave as it's too far out and too variable to sufficiently plan.