In year 13, you project a drop in spending of $5100. For limited time expenses its more appropriate to say you need $60k, then its done. Its immune to inflation and not part of your core budget. If you apply the 4% rule to $43,500 you need $1,087,500. If you apply it to ($43,500-$5100) you need $960,000 plus $60,000; $1,020,000. That's a $67,500 discrepancy, not too shabby of a buffer. Its just a different perspective, plotting your budget with time and accounting for inflation leaves year 13 with over funding.
Back to inflation; CPI is the measure of everyone's inflation including spendypants and frugal types based on buying a set basket of goods. In my inflation beating way, I guarantee that electric will never outpace inflation, if it does it bolsters the case for solar panels and then I'll lock in 25 years of power; similiar to locking in mortgage costs. In fact, if panels keep decreasing, then I'll beat inflation anyhow by decreasing my electric costs. Its a perk of being rich, you have the opportunity to seize upon expensive opportunities, its a lot easier to beat inflation when you're rich. That's a single way to do it, more options exist.
Since hitting semi-retired this year I've noticed several expenses decrease. I predict that 5 years hence my total expenses will be less than last year (when I worked). Your budget is similiar to mine ($30k/year) except I broke out $5000 in education savings (limited expense), I don't have $6000 in health insurance (CDN), my utilities are $3,300 ($1500 less) and otherwise we are close.
I also put a $3000 slush fund into the system, I hate calculating too closely. The last several years I was well under $30k when I removed daycare and education savings (daycare dissipated upon retiring and the education was a simple lump sum set aside and trickled in over years for tax efficiency).