I am wrestling with this also for dd (aged 14.6), and am thinking of doing the following:
certain household chores are to be expected and come with being part of the family:
keep bedroom and bathroom reasonably tidy,
make bed daily,
feed / water dog daily,
feed fish daily,
take out rubbish,
set & clear table,
load and unload dishwasher,
put away clean clothes,
restock loo rolls each week for all bathrooms
Like you, I am starting to get regular requests for pocket money, so when school resumes, think I will give her a monthly amount to cover
i-tunes, apps for phone, etc
magazines/makeup/nail varnish, etc
miscellaneous non-essential purchases (random piece of clothing she wants but doesn't need, endless rubbish from Accesorise, Lush, etc.)
Going out with friends to cinema, Starbucks, Bubbleology, etc
I was thinking of £60 per month (£15 per week), and will revise as needed. I also intend to begin a new rule: if she loses something, I will replace it once. After that, it is not an accident (but carelessness), and dd will have to fund replacements (i-phone earbuds got me started on this line of thought).
FWIW, I will continue to pay for
school clothes & shoes,
basic weekend clothes/coats/shoes,
top-ups on her Zip card for bus/tube/train
basic phone expenses.
I want dd to have enough disposable income that she has a bit of freedom, but also she needs to realise there is a limit so she can budget her outgoings accordingly. Also, by making her responsible for replacing lost items, hopefully she will become more careful in future.
If anyone has a better idea / different philosophy, I'd love to hear it.