how much ££ after bills are paid

hi all well im having baby number 2 and im thinking of going part time we can afford it with paying all the bills but i wondered how much you think a family of 4 will need for the extra stuff i know the more the better etc but as i say im hoping to go part time and i estimate we will have around £900 for food clothes holidays and things like car maintaince etc what do you all think is a reasonable amount??