Oh, totally, there are a lot of variables, especially for those of us who are early in the process and not making killer money.
I make conservative assumptions and plan accordingly. I have a spreadsheet, and as I go along, I'll adjust accordingly. That may mean my time to FIRE goes up or down, but I can assume I'm in some kind of ball park.
"Will our salaries go up? By how much?" I assume 2%/year. That was the new standard at my old job (down from 3%) and is the standard at my new job. I am hoping that here or there I will get something higher for the work I put in, but I can't plan on it, and even if my bosses think I deserve it, they can't necessarily get it for me, so I just budget 2%/year increase.
"What will the markets do?" No one knows! You can use historical averages, but past behavior does not guarantee future results. It seems like MMM often uses 7%, I go more conservative and assume 4-5%. Some people might even argue lower, or higher, that's kind of up to you. Also depends on your asset allocation.
Re: rental properties, I have nothing, it's not really a part of my FIRE plan. In our area rental properties are often more trouble than they are worth, plus they require research and time that I'm not interested in spending.
"How many scholarships will our kids get for college?" Who knows? I would suggest setting aside a certain amount for each kid in 529 plan and the difference is up to them. If they get scholarships or go to state schools, they should be okay, if they want to go Ivy League it's up to them to take on the student loan debt required to accomplish that. I say that having no children, so it's a lot easier for me...YMMV.
"Will we figure out how to grow half our food?" Yes. Budget in some trial and error in the early years to figure out what things grow best in your area, keep best in your pantry/freezer, get eaten most by your family. We grow more because we think it's cool than for anything else at this point, our first year the spinach tanked and we planted way more parsley than we could ever possibly use, so 20% of our little garden ended up useless. But our broccoli did really well, and we eat it a lot, so this year we grew a lot more of it. Play around for a few years and get a sense for it. (unless you are thinking of just buying a farm for FIRE, I know nothing about that)
I don't really use the Super Simple math post, because of the variables you mentioned and others. I use a spreadsheet and make assumptions. My columns are basically YEAR, SALARY, 401k, HEALTH, NI (after the previous & taxes), Amt to IRA, Savings, Cost of living.
I take what I know my current expenses to be, and assume a 3% increase on the cost of living line. Inflation hasn't been that high for a while, but again, I go conservative. I just updated it Friday because the 401k is going up from $17,500 to $18k/year. That also changes what's going into my after tax savings. Based on the SSM, I could retire at 49. Based on my spreadsheet, I'll need until 52.
We're planning on moving to a new house in a few years, so that could change my cost of living. I could get a better raise, and that could up my savings. I'm currently assuming I'll need an additional $6k/year in health insurance premiums once I retire, but that could be way low or way high. As time goes on, I'll get more information and adjust accordingly. Maybe I will be able to retire at 49. Maybe it'll take until I'm 55. But I'm planning as best as I can with the information I have now and that's all you can do, really.
