The best way, IMHO, and the way I did it:
1. Figure out what you are actually spending now on everything. I used Quicken and made sure every penny that went through my life also went through Quicken for several years. There are other financial tracking tools out there now that can do similarly.
I'd recommend several years worth of data, but at least six months at a bare minimum.
Another way is to make sure every cent spent goes through a small set of accounts, then you can take the balance at the beginning of the period and the balance at the end of the period and all the deposits and do the math that way. However, this may not account for taxes that may be withheld from your paychecks before the net amount is deposited. And taxes were the single largest expense category that changed for me between working and FIRE, and I suspect it is one of the largest for most typical people.
2. Go through all of the transactions or by category and decide how much that expense will increase or decrease in FIRE. For me, I knew taxes would go down, I knew healthcare insurance would change, and I knew my child support would end. I thought auto fuel would drop but it did not. I did not think eating out expenses would drop but they did.
3. In your case, you might want to construct different scenarios: Scenario A is FIRE to country A, scenario B is FIRE to countries A and B, scenario C is FIRE to country C. Go through step 2 for each scenario.
4. Then take those scenario FIRE budgets and multiply by 25. You can get a rough guess as to how long it will take you to get there. So maybe you can retire to country A in 6 years but country B would take 10 years. This in turn might inform your decision as to which scenario you choose, and thereby narrow your target FIRE budget to one of the scenarios in particular.
In my observation and personal experience, taking actual current living expenses and adjusting them in a reasonable way for FIRE changes has turned out to be an adequate level of planning for myself and other FIREes' situations that I'm aware of. It won't be perfect, but it'll be close enough to be workable.