SINKs in a second-tier city in Japan. We spend $4,600/month on everything, including business expenses, and taxes and social insurance premiums, which are based off income. Take those away and our monthly spending is $2,500.
Looking ahead, we don't expect to spend the $1,100/month we've spent on travel and outings over the past several years, so knock that down to $100 and we're looking at $1,500, which is basically rent, utilities, food, and the odd medical expense (but not the cost of insurance, which, again, is income-based).
Obviously, a long-term plan would need to include medical insurance and increased medical expenses as we age (we're in our 30s), but I'm a bit surprised that it boils down to $1,500. Probably more like $1,600 in Pandemic Mode; like many here, we've stopped bargain shopping at the grocery store and just grab what we need/what's there.
I am never sure how to calculate everything due to my husband's work: he has a business expense-related credit card and all our phone/internet/car expenses, including petrol go on to this.
I'm a sole proprietor and have faced the same problem. I think the answer depends on what you want your calculations to help you do or understand.
We do separate calculations for business and personal spending. We want the business calculations to show us what we need to reduce our tax burden, and we want the personal calculations to show us present spending and help us project post-FIRE expenses.
Since FIRE for us means me not doing the business any more, our solution is to ask the same question about every expense: "Would we have made this purchase if we didn't have this business?" If no, we count it against the business. If yes, we count it as part of our personal spending, even if it is a legitimate business expense.
Examples:
-I have a business website for which I have to pay various fees. I'm letting the site go when I retire, so that's not an expense we'll need to account for in the future. Business website expenses = All business
-Our work stations take up around 50% of the space in our small apartment. However, we want this same amount of space when we FIRE, so our rent will probably stay the same. Rent = All personal (though obviously we write off the 50% as a business expense for accounting purposes)
