What I have done is create a spreadsheet of all my spending categories and 'average' spending for each. Then I identified which categories are pre/post fire.
For example, I need life insurance only until fire, but I expect to need the same amount of food.
For my car, I assigned a rough value of ~$150 a week ($7800 a year) for pre-fire expenses, and $50 a week ($2600 a year) for post-fire costs. Why? Because I wont be driving 130 km a day for work anymore (queue the clowns).
Current mortage, $500 a week pre-fire. $0 a week post-fire.
Child care: $400 a week pre-fine, $0 a week post-fire.
Then, I went and calculated out what income I need for post-fire expenses, and back calculated the needed stache size (~$850,000).
Now, we finally get to the savings rate. And guess what, buying cars, paying your mortgage, subscribing to TV, and other man toys do not build my stache. Only investable savings builds my stache.
Now that you have an understanding of what you are spending, and when you can stop spending on that item, you can create a projection of your FIRE date.
(e.g. my savings rate with mortgage included is 52%, but since the house add to my stach, my real investment savings rate is 29%. Once my house is paid off, my investment savings rate goes up to 52%. I only look at my overall savings rate when I need an ego boost)