YY to writing down incomings and outgoings so you know if you have enough money for essentials and if you have anything spare.
You also need to save something every month for annual expenses like insurances, car tax and MOT, Christmas, holidays etc etc. Do not consider this money 'spendable' on a daily/monthly basis.
The fairest way of sharing money is to use everything coming in to firstly pay essential bills, food, work travel, child expenses etc. Then put aside some money for savings including those mentioned above.
Share what's left 50/50 for general spends including anything 'personal' that is wholy or partly discretional. Eg clothes, work lunches, gym memberships, hobbies, haircuts and beauty treatments. Eg - you could take packed lunch and a flask to work or you could buy coffees and lunches out. But this can add up to a lot and cause difficulties and arguments if you are spending more than you can actually afford.
Similar for clothing - yes everyone needs some clothes, but most people probably buy more than is truely necessary - if you've added up your sums and can afford to spend 40 pm on clothes, buying designer items will cause problems.
Moneysavingexpert has a good budgeting section and list of expenses to consider.