This is a really good post on putting together a retirement budget:
http://rootofgood.com/developing-a-retirement-budget/
Thanks for sharing that, and hi to the 23 people that stopped by earlier today.
As for the OP's "one off spending that adds unexpected thousands", I try to plan for it explicitly and accept that there are unknown unknowns, but when the $hit hits the fan, it's rarely all at once (the HVAC may die prematurely, but it's unlikely to be the same year I am diagnosed with cancer or total the car or...).
Take housing, for example. I estimated I would have around
$1,600 per year on average for major house repair/replacements on top of the normal repairs and maintenance I experienced while working. That covers replacing the roof, painting, flooring, HVAC, appliances, etc.
Fast forward to today, and I'm in the middle of a $9,000 major renovation to replace windows, siding, and partial roof replacement. It's pretty close to what I budgeted. When it comes to things like house repairs, they are largely predictable in the aggregate. You'll have some systems die prematurely, but other systems will last longer than expected (or you can put them on life support to prolong the life).
Also step back and look at the big picture. If you are driving 7 hours to help take care of aging parents every few weeks, you probably aren't out spending money on cruises, international vacations and shopping for hardly anything. Right now, I'm in the middle of overseeing all the house construction, so we decided to skip the December vacation so I could watch over the contractor. Sure, we are spending $9k but we "saved" a thousand or two on the vacation.