Another second of noodling solved it. If it's using the percentage to determine up or down, I can just set them the same. (I really do like to break models and/or use them outside their intended use. Heh.)
So setting both the floor and ceiling percent to 4%, then it will use my spending floor and ceiling. As I said above, it does make your spending jump up and down a lot year-to-year, but it increases the success rate to 77%.
Hmm.. so that creates some interesting situations wherein the success rate goes down as you increase the spread.
In any case, getting back to the relevant stuff we were discussing with Sol's "50% success rate" scenario...
I do like your aggressive nature on that though, and the viewpoint of "50% just means you'll be just as likely to be fine as to need to trim and adjust a little."
EDIT: After two minutes on cFIREsim, I found that 50% success rate is about a 5.85% SWR, which is the same as having about 17X your annual expenditures in assets.
Spending: 46,800/yr
Portfolio: 800,000
WR: 5.85%
Success Rate: 50.44%
Then setting your "fat trim" to be 20% of your budget (i.e. minimum spending of 37440), ceiling spending of 46800, min % withdraw from rising to be 5.85% and max withdraw from sinking to be 4.68%, or 37440/800000), you raise your success rate to 72%.
So variable spending will help, but historically even if you cut back 20% of your budget due to bad years, you'd still have run out of money 1/4 of the time.
So one may need to be even more aggressive than that in cutting, or shoot for a higher success rate than 50% initially (i.e. lower than 5.85% SWR).
In fact, if I run a scenario of 5% SWR...
Portfolio: 800k
Spending (5% SWR): 40000
Fat trimmed spending (20% cut): 32000
Normal success rate (at regular inflation adjusted withdrawals, not variable) is 70% (69.91)
Now we switch to variable spending. Max spending 40k, min 32k. 5% and 4% ceilings. That increases your success rate all the way to 92%.
So if a SWR of 5% is viable over 9 times out of 10, historically, with some flexibility in spending, 4% is quite ridiculously safe, if you're willing to trim fat in bad years.
Starting with a 5% SWR and dropping spending by 30% in bad years (5%/3.5% and 40k/28k ceiling/floors) gives you a 99% success rate (just one failure, 1966).