I’m looking to FIRE at about 40,
At estimated 40k/year FIRE expenses
I'm 30 now.
Pension ... I'm conservatively estimating it at 11k/year starting at 62.
Right now I'm saving about $18k a year
Roth: $50k
Tax deferred: $0
Taxable: $38k
529: $23k (I know i know, it was before I found MMM and I am not contributing anymore...)
HSA: $2k (from an old HDHP, no more contributions)
I think the reason you're not coming up with an answer that doesn't make sense when trying to figure out how to save enough in Roth/taxable to last five years and still contribute to traditional is that you're just not saving enough period. Ignoring the pension you'd need $1,050,000 to support $42k ($40k spending + $2k taxes) at a 4% withdrawal rate. Even if you say you took the pension from day one, you'd still need $775,000 to support $31k in addition to the pension at a 4% withdrawal rate.
By my math, with $50k + $38k = $88k of current retirement savings, a 7% rate of return and 10 more years of saving $18k you'll get to $422k by the time you're 40.
Those are the (some might say overly) optimistic withdrawal rates, and rates of return some on this forum would use. Personally I would use a 3.5% withdrawal rate (see
https://earlyretirementnow.com/2016/12/07/the-ultimate-guide-to-safe-withdrawal-rates-part-1-intro/), a 5% rate of return (7% is the historical average, some slightly pessimistic people like the founder of Vanguard are projecting 3% given the current high valuations).
I also probably wouldn't consider the pension, just as I don't consider social security. You could play around with
http://www.cfiresim.com/ but I'm guessing the difference between the amount needed to last 22 years before getting an $11k pension and the amount needed to last indefinitely without the pension is not that much. Also 32 years is a long time from now, and who knows what might happen to your employer between now and then. Even some government pensions aren't too healthy right.
So based on my (some might say overly pessimistic) more "realistic" assumptions, I'd want to see you maxing out your 401(k) ($19k starting in 2019), your Traditional IRA for both you and your spouse ($6k each starting in 2019), and adding $53k to taxable for a total annual savings of $84k, which would give you $1,200,000 10 years from now, which would support a $42k/year withdrawal at a 3.5% withdrawal rate. Given that you'd have more than enough in taxable to handle getting the 5 year Roth conversion ladder started.
If you keep going at $18k/year you're looking at more like 25-26 years based on these same assumptions (although at that point counting on the pension might be more realistic, so you might be able to shave a bit off of that). In this case you will want to put some in roth to last the 5 years until you can withdraw from tax deferred penalty free, but it doesn't need to be the whole $18k since you'll be saving a lot longer.
Either way, this is why the typical response to people asking "how will I have enough to last the 5 years to get the Roth conversion ladder going if I max out tax deferred?" is "Don't worry about it, to reach FI you'll need to save way more than your tax deferred limits anyway, so you'll have plenty in taxable." This isn't always true (in my case I have a ton of available tax advantaged space for a couple of reasons, so I need to think about it a little more carefully), but it is true for most people.
Before you get discouraged, realize that $18k/year is awesome, and puts you way ahead of most the country. Retiring at 55 also puts you way ahead of most of the country. The thing is, mustachians are the badass special forces of retirement saving. There's no shame in not keeping up. It takes sacrifice, hard work, and not a small amount of luck and natural talent. Either we live on half of your $18k savings a year and so we need hardly anything saved, or we save your $18k (or more) in a couple of months, so we can get to a $40k spend (or more) relatively quickly. Lots of people just don't think it's worth it, and some who do still might not be able to get their for whatever reason.
If you want to try to get to mustachian levels of spending (lower) and saving (higher), then posting a case study as you planned to would be a great idea! In the meantime, don't worry about having enough for the Roth conversion ladder too much. Either you'll be at this a lot longer than you thought or you'll be saving so much that it just won't matter.