I think this goes to the "live like you're rich" thing the blog talks about so much. Once you've really adapted to preferring to do things yourself; to finding luxury goods silly or off-putting; to wanting a smaller, sparser living environment--you don't need to watch the budget in the same way, because you will find yourself spending less regardless.
Personally, I'm not remotely there ;) though some of it is really creeping up quite well (I vastly prefer biking to work over driving, and I find restaurant prices more and more irritating, especially for alcohol). I can definitely see a future where my "budget" will be kept low purely because I don't want to buy most things, but at the moment, nah.
So for me, I do have to draw a sharp line between needs and wants, and I have to re-evaluate that line. For instance, I've been getting 95% of my books from the library (and I read a shit-ton of books), but then my ereader died. Is a new ereader a need? Probably not, but reading is pretty close to a need for me, and I spend hours a day doing it. So the actual ereader I bought was definitely a want, but it connects me to something I'd call a need--that was a tricky one for me.
To be fair, though, that can be broken down into the much more boring and endless "do you really need to buy chicken when lentils exist" or "do you really need to have more than one shirt" kinds of questions, which gets into a frankly less useful and less interesting or inspiring way of looking at it. Better, I think, to try to go back to the lens of: what about this is going to make me happy, and isn't it true that most of the time, the cheaper/free option will actually result in greater happiness? Not necessarily lentils > chicken, but learning to cook > never learning to cook, or learning to take care of nice clothes > burning through random ones carelessly.
And if you can learn to see it that way, you find yourself in Arebelspy's shoes, where your wants are very few and very affordable. That's the dream!