I would recommend tracking it both ways, if you can. I think the most important thing is to track your normal budget so you can confirm you are saving what you plan and avoid lifestyle creep.
However, there are a number of big expenses that look like one-offs individually, but if you look over a period of time you can see that *something* happens every couple of years, so you really need to account for that. E.g., we did a big "back half of the house" remodel @ 10+ years ago and thought, hey, that's it. Then a few years later we said, you know, we really need to re-do the powder room. Then we spent a few years replacing all the windows. And last year we finally built a garage. Now we are thinking about the upstairs bath, which hasn't been touched since about 1982 (and totally looks it). And let's not even talk about the replacement vehicle(s) we will probably need within the next couple of years. . . .
I am assuming your situation is likely different (we knowingly bought an old house; the extra spend is the price of admission for a house I adore). But the larger point is that if I were just looking at my "normal" spend, I'd be thinking we're doing great -- but I'd be missing a very significant chunk of money that we are spending every couple of years, and that is likely to continue to recur in some degree after we FIRE if we stay in this house. So my prudent FIRE planning involves looking at those "one-off" expenses and finding some sort of anticipated average based on past history -- honestly, I probably don't need to consider the first huge remodel in that analysis (that one caught up about 100 years of technological advancements and will not recur in any way, shape, or form), but I can count on other, smaller-but-significant expenses continuing. They will be different expenses from the past -- the bathroom won't need re-doing, but we may need a new roof at some point. But history suggests that things will continue to need repair/replacement, and that I will continue to come up awesome ideas for things that "have" to be done, even though I don't know what they are right now because I haven't thought of them yet.