So there are two main approaches
1)integrated into an overall budget
2) just its own thing.
I do the second - I come up with an annual amount (which I'm not rigid about, $50 over is ok) and it is based on what I think I'll spend that year plus a little over for things I haven't anticipated.
My experience has shown that every few years I have a bigger expenditure and have to either
1) replace a lot of things
2) have a chance in circumstances - e.g., going back to work
In between years I have a lower budget in maintenance mode - just buying a few things that need more frequent replacement or refreshing.
For me, footwear is a major part of my budget, as I'll pay higher prices for comfortable and fitting shoes - so the amount of shoes I plan to buy affects the amount I'll put aside. (Last year I budgeted for a couple of shoe replacements but didn't end up needing them. Bonus!!)
I'm an underbuyer, and not someone who enjoys shopping so for me a budget is giving me permission to spend that much. For many others a budget is a restraint.
My budget is pretty small by YLF standards. I try to only buy what I'll wear out, and don't like to churn.
As to amount per item I guess I have ideas in my head about how much I'm willing to pay (again, more for footwear). They've gone up since I joined YLF!!
As for the % of pay issue, I had heard the 5% recommendation before and went down a rabbit hole of reading up on it! My initial reaction, unlike LisaP and Bijou, was that it was a lot (FI, I'm with you on your reaction) I did a rough calculation - I spent around 2.3% of my take home pay on a big (not maintenance) year. The 5% is supposed to cover the whole household though, but I'm sure the daughters and my DH wouldn't spend 5% of his earnings (even though I work part time, and he doesn't have full pay, so our household income isn't super high)
But then again, our needs aren't great - I don't have a high profile job and DH can work in a tee and jeans.
I did for a few heady weeks in 1997 spend nearly 1/6 of my take home pay on clothes - yes - the first weeks of a job. I cut it down once I had a working wardrobe.
One last thing - I get a voucher through my credit card every autumn for either a department store or a mall and I found that I can buy quite a bit of clothing through that.