The fairest way is for joint expenditure to be pooled but for each person to have an equal amount for free spending on themselves. How this is practically divided up is up to the couple, there are a number of ways that this can be done.
So joint expenses would be rent/mortgage, bills, child expenses, groceries, holidays, white goods etc, emergency savings, cars as needed and within budget, and fuelling and maintenance of said cars, work travel.
After joint essential expenses are covered, each party gets an affordable amount for their own personal spending and anything personal and discretionary comes out of that - clothes for adults, hobbies, drinks with friends, books, magazines, that sort of thing.
Of course there is some overlap and room for interpretation that the couple would need to agree on, and that depends largely on how much disposable income there is. If funds are short, a lot more control is needed but if there is a lot more disposable income, things can be more relaxed.
I'm thinking of things where the amount spent can vary hugely and people have different ideas about what is reasonable, eg
Mobile phone - essential but should come out of personal money - what if one person is happy with a basic phone and a tenner a month contract, but for the other, only the latest smartphone with all you can eat data will do?
Cars - small, cheap to run, second hand and kept for ages, or new, sporty and replaced every 2/3 years?
Work lunches - leftovers from home or daily trips to Pret or similar?
You can't say that a car, work lunch, or mobile phone is a 'joint expense all out of the same pot' if one person is spending £100 pm on these items and the other £1000 pm'