We went through reasonable expenditure over a year based on current lifestyle & then divided it by 12 so DD had to learn to budget but had this original to use as a basis.
Good idea to both keep a copy of the original list of what was agreed in case of queries over what was supposed to be covered.
So much depends on the 'child' & how the family operates re things like buying presents for friends/family, socialising, mobile phone bills ....
Some families differentiate between essential expenditure & optionals - so parents buy school uniform, books, equipment etc - while others include an allowance for this. I dont know what happens if the child doesn't then have the funds when they need new school shoes, blazer etc
School lunches are a bit if a tricky area as I have concerns about teenagers eating properly at lunch time. If the normal arrangement is for DCs to have school lunches then I believe this should be totally separate from other money so there is no encouragement to buy make-up, CDs... instead of lunch.
Of course its impossible to ensure they actually but the school dinner! - except where they have a debit card system/parents can be billed termly etc.
My feeling is that it is better to start on a reasonable but 'low income' which can be increased as the child gets older & their needs develop. Also would start with limited areas of responsibilty & increase these with maturity (eg. winter coats, shoes, school kit only once past GCSE stage maybe?)
Other factor is who pays for equipment for hobbies/ sports. Can be a total minefield wher DCs are sporty/ musical.
And school trips - optional v. curriculum?
Maybe we should start a 'check list' of all the items to be considered