Do coast FIRE calculations apply only to the individual?

Any type of financial planning (especially retirement) applies to the entire household - not just an individual. If you have kids, you need to consider things like life insurance and college savings. If you're married, you need to consider all sources of income and all living expenses.

There's not a one-size-fits-all formula. The formula that is given is just a simple future value formula that assumes a constant growth rate. You have to take many more variables into consideration. Your partner may not want to sacrifice comfort now to retire (or coast) early, or they may want to fully retire early even if it means working harder a bit longer. You have to decide that as a family.

The hardest part is not the math - you can talk to one of the financial advisors that conveniently have contact links on that page, or find someone locally to help you through the rough math. The hard part is coming up with a reasonable plan and sticking to it. It's easy to plan for the best and have a bare bones budget that gets you to retirement early, but when you have to replace a car, or want to get away on a fancy vacation, or have kids with braces, those plans can quickly become unrealistic.

I've known people where one spouse "retires" early completely, some where one "coasts" with a lower-paying job, and some that work full time until their late 60s, enjoying life with their young family. Each have to make sacrifices at some point to accomplish their goals, and none of them are "wrong".

So one option is just to double the numbers (The nominal numbers, not growth rates) and assume they apply to both of you equally, but that leaves out a lot of variables.

Picking "The Number" is non-Trivial

The article you link kind of glosses over how much you need, likely because it's personal and reasonably complex to figure out -- but it does give you the general idea.

You need to figure out how much you expect to spend per year in retirement - where "you" is your family unit. Are you taking care of disabled kids? When will you have paid off your mortgage? One spouse has more medical needs? Everything goes into the annual cost.

The article you link starts with $100,000 per year, but your bio says you're in San Francisco so you'd likely need much more than that.

Then you multiply what you need by 25, and you get the size of your family's nest egg. For example: 25 x $250,000 = 6.25 million -- means if you need 250k a year to retire, your nest egg has to be over 6 million.

A Year Makes a Huge Difference

There's a lot of uncertainty about what you will need to live off 20 or 50 years from now. Luckily, how accurately you predict that number many years before retirement doesn't really matter. What matters is that you have an accurate number when you do retire.

For example: since the market's average ROI is 6% (after inflation and taxes) if you have 6.25 million, but realize you need 6.5 million to retire, that's only a couple extra months of work. 6,250,000 * 6% = 375,000 and 6.25 + .375 = 6.625 million means delaying your retirement by a year gives you more than enough to close the gap -- and that assumes no additional contributions, just growth of principle.

FIRE is about Cutting Costs

The reality is that most people old enough to think about retirement can't really change their income much. If you're looking into FIRE, you are almost certainly an upper middle class white collar worker. Perhaps getting a graduate degree (or another graduate degree) might increase your earnings, but your ability to pull in more money is limited short of starting your own business with all the associated risks.

So, if you're looking into FIRE, you're looking into cutting your costs. You are trading things today (a nicer house, better computer, whatever) for time later (retire x months / years earlier).

So I wouldn't waste too much time trying to get a super accurate number to shoot for. Get a number that's good enough to help you decide "Yeah, making coffee at home every day instead of hitting the drive through will let me retire nearly a year earlier, and that's enough to make it worth it to me."